I just talked to someone at Aerojet and they gave me some info
interesting enough that I thought I would post it.

For the ISP range that I am interested in, like 1,500 to 2,500
a Hall Thruster is the way to go. For lower ISP like 500 to 1,000
an arjet would be good. For higher ISP, maybe 2,500 to 10,000+
an Ion Drive would be the way to go.

They have a 4.5 kw Hall thruster that is 20 cm in diameter,
produces 285 mN thrust, and weighs 12 Kg.

A Hall Thruster is a circle where the thickness does not change
much with the power level. However, the area scales linearly
with the power level. Also, the thrust scales linearly with
the power level, though efficiency goes up a bit at higher power.

They have built a 50 kw Hall Thruster for NASA. A system
with this thruster might be like $1.5 to $2 mil. Most of the
cost is actually in the electronics for power conditioning and
control. There is nothing fundamentally keeping them from
building larger ones, though there would be some R&D cost to
do so.

Ion drives have more complicated electronics and higher voltages.
For the same power level (not thrust since higher ISP) they
probably cost 25% to 40% more.

They have not had trouble getting export licenses but it takes like
6 month to get them.

I asked how much cheaper the thruster would be if I totally relaxed
the reliability requirements and ordered 200 of them. The answer
was maybe 1/4 the price.

----- now on to my comments ----

Scaling from the 4.5 kw unit, the 50 kw unit must be a bit over
3 N of thrust. This is the largest commercial Hall Thruster I know of.

My gut instinct is that there could be more than a factor
of 4 reduction for the due to relaxed requirements and volume.
I forgot to mention that I would relax the weight requirements,
be happy with non space-rated electronics, etc.

Since most of the cost is electronics, and electronics tend to get
cheaper fast, the cost of these thrusters should be dropping over time.

-- Vince