Thrust of a rocket/engine is given by
$T=\dot{m}u_{e}$
where,
$\dot{m}$ is the mass flow rate of the exhaust of the propellant,
$u_{e}$ is the exit velocity of the propellant.
So the mass flow and exhaust velocity for same thrust levels are inter-dependent. If you want to slow down the exhaust velocity, you will have to increase the mass flow rate for the same amount of thrust and vice versa.
The basic premise behind the electric propulsion is this one simple equation, you couple more electric energy into the propellant to increase the exhaust velocity so that you can increase the propellant utilization(meaning for the same mass of fuel you get more thrust over time, which is Isp).
If you are interested in MHD propulsion or EP in general, I would recommend you to read a book by Robert Jahn, Physics of electric propulsion.
Updated after edit:
I really wish the answer could be simple as the question. First and foremost, In space nothing is less durable. Everything is rigoursly tested, e.g. NASA/UMich's X3 hall thruster is being tested for endurance, they will fire it continuously for 10000 hours, by my estimate the cost of propellant should be in order of 10mil USD. It is one of the high power thruster that currently exists, it can take around 100 kWe(kilo-watt electrical).
To come to your actual question, it is not a simple equation, there are very complex scaling laws involved for such thrusters. Thrusters have different efficiency factors like thrust efficiency, thermal efficiency,etc. Even the power unit(PPU) has an effeciency. So the thruster selection is done mission-by-mission basis, it all depends on how much power is available, the delta-V requirement, mission and budget constraints.
Still on a whole, small thrusters can couple so much of electrical energy into the propellant they don't compete with higher power thrusters. To put things into perspective, some very low power arcjets around 100~300 We(watt electric) produces thrust in ranges of 10~50mN(milli-Newton) depending on propellant and similarly the thrust efficiency varies from 10~35%. These types of thrusters have exhaust velocities(Ce) typically around 1000 to 10000 km/s(depending on propellant and power.
On the other hand high power thrusters have exhaust velocities well over 15000km/s (depending on design, power levels and propellant). I think now I have equipped you to dig deeper. If you still are curious, please don't hesitate to ask.