What would be the best engine to put in a 77 280z that would cost under $2,000????? (with the engine in decent condition) How cheap can I get a nice RB26DETT engine 4.

Where are you?

If in NZ RB20DET's are $1600NZD, RB25DET's are about $2800NZD if you can find one, RB26DETT's are heaps, never looked for one, nor seen any advertised (probably very hard to come by) but I have seen people talk in the region of $4500 but Cyong would be able to give a better price. this doesn't include the other expenses involved, you'd be hard pressed to do an RB conversion for under $2000

If your in Australia the RB30's would probably be cheap, failing that American / Australian v8's or L28 turbo's would probably be the cheapest option depending where you are.

I saw a couple of Nissan V8's for sale recently but they were over $3000