ok, well piggy back you'd be looking at uni chip, smt6, greddy e-manage, probably something from haltech or microtech. all of these would range from around $600-800 and upwards
i was reading on a website about r31 skylines that if the plug of an ecu matches another ecu, then the ecu's are fully compatible with that car, if this was tested or not i dont know
full replacement aftermarket ecu's are not worth it in my opinion. people will argue with you either way until they're blue in the face, but a lot of fully aftermarket replacement ecu's just dont have the same ability as a stock ecu. for these kinds of things at the absolute cheapest you'd be looking at $1500 or so.