I got my 200-4R with torque convertor, TV cable and cable bracket at the local pick-n-pull for $70. I pulled it myself so I could check the year (after '86), application (V-8 4 bbl car), and condition (mileage on car, fluid smell, amount of friction material from wear in the pan) before purchasing to make sure I got a good one. There is a tab on the back of the transmission that has the application info if you are wanting to buy one that has already been pulled.

You can leave out the lock-up on the 200-4R (same as the TH350 then) if you want to keep the cost down.

That just leaves the crossmember as the biggest difference. My car already has the TH400 crossmember so I didn't check any farther, but I'd suggest getting the crossmember from the donour car and doing a trial fit with that. Very likely the ends where it bolts to the frame will need modifying (width, fore and aft bolt placement, etc.) but if you take the new one to a good welding/fab shop with good measurements you should be able to get something usable for a reasonable price. I'd be surprised if they take more than 2 hours of shop time to do the job. And that way you can keep your stock crossmember as well!

Get a later low mileage 200-4R that's factory built for a V-8, pull the pan to see if there is any debris inside, change the filter and fluid while you're there, get the crossmember modified and bolt it in. That's about the cheapest way that I can think of to replace the ST300 with a better transmission; others may have better ideas!