How cheaply can I do a good rebuild for?
 

so I've blown up YET ANOTHER motor.....

I've got 2 blown s5 jspecs in my garage... both should have good housings but I know at least that between the two I'll have enough parts to build one decent motor.

How much $$$ to build one, on a budget?

If everything is ok, you'll just need a gasket set. Try Atkins Rotary.
The last time I checked it was about $320. for turbo gasket
They had the best prices and have never failed to deliver what I need. And I live in the middle of the Pacific. When you take it apart you should mic the housings if it is within spec. How did you blow the 2 motors??
Also, invest in some Hylomar sealant for all the O-rings.

From experience, I highly doubt you will be able to do a decent rebuild for under $700.

-Joe