4.10's
 

how much to buy and install

Nice. 270 for gears plus install kit plus gear oil.

250 for install on average.

Depends, are you just looking for the gears, or do you want a package deal with a tuner. (<-- Highly recommended)
Are you doing the install or are you hiring someone to do it for you? Are you sure you want 4.10s not 4.30s?

Typical amount spent is anywhere from $200 - $250 after shipping just for the gears...$300 for gears and speedcal. But if you want the gear and tuner (<-- again - highly recommended) it'll cost you anywhere from $550 - $600

Installation costs vary from place to place...usual price is anywhere around $150 to $300, unless you do it yourself. Either way you'll probably want to get the bearing kit which usually is around $80 after shipping because it makes the whole transitioning between old and new a lot easier. It also helps prevent gear whine, however if installed improperly, gear whine is inevitable.

4.30's? at what point do gears become completely impractical, i dont mind having to shift alot what do you reccomend

4.30's in a 302 car is kinda overkill, I beleive you have some kinda fox body right?

87 gt

Before you go and get gears I would figure out your plan for this car. If its gonna be a drag strip monster, a street/strip car, or a cruiser/highway brawler. You also need to know if your gonna do some kind of power adder etc....

street/ strip

What about power adders? If you go turbo I wouldn't even swap gears because you will want them to make the car hold each gear longer. If you go nitrous you will want more gear then say if you went with a blower. Like me if I was going to do a nitrous car I would go 3.73 maybe 4.10's in a fox on spray. If i went with a blower I would go 3.73 most likely. Its not like a mod motor where you need ass tons of gearing. So figure your shit out then figure out what gear you want.