N/A heads work just fine and are rarely cracked. There are a few myths about 2.3t's that have become accepted as truths, with nothing to really back them up. I won't get into that though - that's off topic in here.
IMO, if you're talking about cost, you're talking about your ability to tune, because that's where the power is. If you're smart enough to tune something effectively so you're not paying someone else to do it, your hp/$ goes way, way up.
Beyond that, it's a matter of finding out what can hold the greatest amount of power in stock form. For example, 5.0's have good pistons, but the block is weak. Much bigger motors may have stronger blocks in stock form (and I keep saying stock because that's usually the cheapest way to go), but the pistons may not be as good. So you get back to your ability to tune, since even GN's didn't come with forged pistons, I'm pretty sure, indicating that the ability to achieve and hold a solid tune is where you'll find the power.
Also, a lot of people forget that there's a lot of power in higer revs, so that's something else to look out for. That becomes a tradeoff, since bigger stuff doesn't always like to rev (again, in stock-ish form).
So I'd say find the biggest cheap thing that will stay together and boost the crap out of it. Look in magazines like Car Craft and you'll be stunned at what people are doing.
http://www.carcraft.com/featuredvehicles/ccrp_0711_1988_pontiac_firebird/index.html
I know you probably weren't hoping to see a Firebird, but you can't deny that's it's pretty badass, not to mention very grassroots.