Help me out....
 

Going to buy a 3rd gen rx7 i test drove it and when i would shift into 2nd gear the rear would make a noise like DUNK, and the shifter would sink in and pop back up can somebody tell me what the hell that could be...thanx

Possibly a cracked/broken Power Plant Frame (PPF).

How much does that cost to fix...

The PPF itself will probably run you between $300-400. If you pay for labor, you can probably count on another $300 or so. If you do the labor yourself (assuming that is the problem), you can check this thread:

https://www.rx7club.com/3rd-generation-specific-1993-2002-16/power-plant-frame-install-114899/

now is this thing big or small..and is it easy to install....sorry for the stupid quest, but i want 2 no b4 i waste 11,500 for it...and would it be worth buying if this piece is broken..thanx

I can't tell you if it's easy to install. What may be easy for me, may be hard for you. There are worse things to replace on this car than a PPF.

The question I would have is if the PPF is broken or cracked, what else is damaged (i.e. motor mounts, differential bushings, etc). You should have a qualified mechanic who knows these cars look it over before you put any money on it.

Dam there's no mechanics out here that no bout these cars...dam i want this car so bad there so hard to find.....

Most likely a cracked power plant frame and or a broken motor mount and bad differential bushings. They are all relatively easy fixes. The power plant frame can be welded or you can find them used fore $100-200. Motor mounts are expensive, they run about $250 each. For a less money you can get some aftermarket urethane ones .If you buy the car I'll be happy to look it over.

thanx guys...i love these cars there so hott.....