would be tough to beat 400 for a complete a15 these days i think, just know that you will have to rebuild it and the cost of the motor itself will be a small fraction by then. In the end you wont even care what the initial purchase price is.

Most Datsun 1500 utes had the J15 - hope you got a good look at the motor and are sure it is an A15! they are visually different looking motors but its worth checking

just had an idea - maybe you made the mistake of trying to pay with money - maybe you will have more luck with alcohol or shotgun shells or something - roadkill maybe?