That's a hard one to answer, but the price typically relates directly to condition.

These were the latest for sale on here I think, and quite a good price, minus the manifold, air filter housing etc.

Twin SU type Hitachi's

I've paid $500 for good condition SU type twins and manifold. Without the air filter housing which are hard to find in good condition.

I vote to work the A12, get some experience on how it all goes together. Then upgrade to an A14 or A15 once the cash is back up and transfer the goodies onto the bigger A14 or A15.