David Hammen wrote in a comment on Why there is not a single orbiter for Uranus or Neptune? that:
The two Voyager satellites still receive multiple Deep Space Network contacts per week, each of which costs several thousands of dollars to tens of thousands of dollars. Then there are people. The most recent Voyager Space Flight Operations Schedule lists eleven JPL employees. I'd be shocked if a fully-loaded FTE JPL engineer costs less than $250K/yr. Plus there are researchers in other NASA centers and in universities who are supported by the Voyager budget.
Let's go with $10,000 per contact for the Voyagers, via the Deep Space Network, to have a figure in the middle of that range. (If someone has a better estimate, by all means feel free to use that for an answer.) At the low bit rates involved, a contact might last for a few hours. I realize that the equipment is awfully expensive and probably takes a small army of people to maintain, but that still boils down to a few thousand dollars per hour of radio time -- pretty close to a dollar per second. Note that this is in addition to the people actually working to directly support the Voyager missions.
What makes using the DSN so expensive?
A rough costs breakdown would likely make a perfect answer to this question, but a general reasoning could pass as well if it is well-reasoned enough.