SR cost, at least $10K maybe a lot more.
A15 cost, buying an engine someone else has already worked up $3K, do it your self $5K+

Modding cars is not cheap, and the less work you do yourself the more expensive it gets.

It's very non Datsun to say but if you want a real fast car cheap, Jap import Nissans are probably the best way to go. Silvia, Skyline.