how much does an engine rebuild cost on a 20vt?

enough to make an exchange unit a better option I would say

It really depends on what wrong with the old unit.

If you have really damaged the bores/pistons, then a 2nd hand engine may well be a better option.

I paid a grand to have the engine top end rebuilt the full head with all 20 valves being replaced (head gasket, etc), and with all the cambelt and tensionners replaced. This was done by motormech, so I would say anywhere from 800-squillions and squiliions of pounds