How much should a 1965 Mustang Fastback w/ 289 V8 cost?
It depends on many factors - but be assured, this is a very desirable, classic pony car and it sounds like it has all of the good options.
If the engine and drive line are original - keep them original. Overhauling the original 289 (if a tired engine is the reason for the swap) is much more desirable than installing a later 302.
The price depends entirely on the seller and the buyer - I don't think that an original one in decent shape would be found for less than $8000 - $10,000. A really excellent one could easily go for much more.