Caveat: Most of my knowledge of such things comes from "word of mouth" discussions with my senior colleagues and their experiences with how things have changed in regards to funding and costs. So take that into account when reading the following.
Several years before I was hired (I was hired in September 2010), my boss was asked to draft up a modern version of the Wind spacecraft. Without launch vehicle, the base model (missing the gamma ray detectors and some other bells and whistles) still came in at over \$300-\$400 million, depending on various design features and types of instruments. At that time, United Launch Alliance (ULA) had a monopoly on launch vehicle contracts with the US federal government. Long story short (and overly simplified), the US government signed a long-term contract with the ULA in the 60s or 70s that guaranteed a fixed cost for each launch vehicle and discounts on bulk purchases. This resulted in the ULA developing a monopoly but it was "allowed" since the ULA was not a single corporation, but a group of them. Note that what I am calling the ULA was not formally formed until 2006 but roughly the same group of companies were involved in the original long-term contracts for launch vehicles.
Anyways, once that first contract ran out, the ULA made shorter term contracts that always increased a great deal and eventually helped end manned spaceflight because each shuttle launch was running well over \$1 billion [total cost, not just launch vehicle].
So what's the point of this background? In the early 2000s, NASA was tasked to implement what was later called full cost accounting in response to their inability to properly tally the actual cost of the shuttle program (among other things). Prior to this form of accounting, funds were pulled from larger "buckets" for various missions to do various things. On the one side, this makes things easier to finish on time and easier to implement changes. On the other side, a lack of proper accounting means it quickly becomes impossible to actually determine the cost of any given project or mission.
As an example, the Wind spacecraft was built as part of the stand-alone Global Geospace Science (GGS) progam along with the Polar. The spacecraft bus for both missions were built by the same company (I think Ball Aerospace comes to mind, if memory serves me) but that company signed a fixed cost contract for two spaceraft buses. That contract was signed in the 1980s, when I was still in grade school, and I very much doubt the company or NASA has any detailed, breakdown of the costs for each bus. The instrument teams have reported to me, by word of mouth, that Wind was the last of the "good old days" missions where the science truly drove everything. If the scientists wanted XYZ, the engineers tried to figure out how to deliver (again, a gross over simplification to make a point). So during the design and build phase of these older missions, things could change a lot and the instrument teams need only to ask for more money. In most cases, according to those that spoke with me, it was relatively easy to justify the changes and get approval for more funds. This further increases the difficulty of tracking costs. Finally, the funds for the mission came from a few of the large "buckets" to which I previously referred and Wind was NOT one of the biggest spenders.
Now we move on to post-launch time frames and the spacecraft needs to communicate with the ground and perform maneuvers. Wind has always relied upon the Deep Space Network (DSN), which has its own line of funding. In fact, even in the latest 2020 Senior Review, DSN time costs were funded through a completely different funding line (i.e., they were not considered part of the mission costs). To be fair, each mission's cost for DSN are appropriately tracked in a separate system called Space Communications and Navigation (SCaN), but they are not tallied as part of the mission costs. In any case, up until very recently, the maneuvers for Wind were planned and calculated by the flight dynamics group at Goddard Space Flight Center. This group operated under a similar funding model to that of the shuttle program, i.e., they pulled from a few large "buckets" and individuals were fully funded on any given project rather than being partially funded by several projects. In short, this led to unnecessarily large costs for even simple maneuvers and Wind has been sitting at L1 since ~May 2004 (well, recently it switched from a Lissajous orbit to a halo orbit but otherwise it's been relying on very simple maneuvers). In summary, designing, building, and operation spacecraft is very expensive and it can be incredibly complicated to track funds especially on older missions.
In what ways can it be argued (by Farquhar or others knowledgable on the subject) that ISEE-3 was "the most cost-effective spacecraft we ever had"?
Okay, so the ISEE-3 (or ICE) spacecraft was launched in 1978. The biggest difference between early missions like ISEE-3 and current missions like Parker Solar Probe is that the early missions did not need to go through all the tests, verifications, safety and mission assurance, etc. However, those early missions also fully funded a lot of senior people for over a decade, which adds up quickly. As I discussed above, tracking costs in early missions like ISEE-3 is nearly impossible due to how funds were used and how people were funded. Launch vehicles were also not considered part of the mission costs like they are now. Perhaps more importantly, in those days the scientists were even less familiar and less involved in the actual budgeting of each part of the mission (e.g., they wouldn't know if 3 or 5 engineers were fully funded to operate the spacecraft). Conversely, due to the reduced oversight, many of the instruments were built and tested in scientist's garages or in their basements. Parts were ordered from any vendor the scientist knew, not ordered through an online bidding war between different companies that have to be approved by a separate department of the government like now.
I do not know Dr. Farquhar nor do I know from where he gleaned such a bold statement. My knee-jerk reaction is to be very skeptical, as I have seen how things used to be funded. I can only speak from my experiences with Wind and my efforts with the team to reduce costs (e.g., do our own maneuver planning and calculations, eliminate unnecessary and unsupported but very expensive software, etc.) and my conversations with folks that have been at NASA since as early as the 60s. So my best guess is that ISEE-3 was most likely, at minimum, what we would now call a MIDEX (it's other name is Explorer 59) but as I said, it's impossible to know how the costs translate to missions today. I do know that ISEE-3 operated at L1 for nearly 4 years, then was sent after Comet Giacobini–Zinner in 1982 (it passed through the tail in 1985). It made a flyby of Comet Halley in 1986 and became a heliospheric mission in 1991. It was eventually ended in 1997. That puts the mission lifetime under 20 years total, most of which was far from Earth (i.e., very low telemetry rates, thus little science data).
For comparison, Wind is over 27 years old and fully operational with over 5800 refereed publications (https://wind.nasa.gov/bibliographies.php). I do not know how many total publications ISEE-3 accumulated but I do know it was nowhere near 5000 (i.e., the space plasma community was much smaller and publication rates in the 70s, 80s, and 90s were statistically lower per researcher). The Hubble Space Telescope has been operating for over 30 years and has accumulated over 18,000 (https://archive.stsci.edu/hst/bibliography/pubstat.html) [though Hubble is nearly an order of magnitude more expensive than Wind].
In summary, it is incredibly difficult to verify or disprove any statement along these lines when actual costs are not known. This statement is vague enough to allow wiggle room by Dr. Farquhar to argue in support of it. However, I am fairly confident most would have a similar initial reaction to mine, namely, doubt of its veracity. Older missions may have taken less time from design to launch, but that did not always translate to lower total costs.