First off, welcome to BP.net!

1) As there is no "standard", really, for BPs (they all grow at different rates, just like humans), I don't think you can pick a good BP by its weight or size. I would make sure that it its body is round (basically, you don't want it to be "triangle shaped" with its spine clearly defined). If you're picking one up at a show, you really shouldn't have any problems with this though. Breeders tend to bring the best of their animals to show.
2) For a pinstripe, I've seen them anywhere between $300-500, depending on if its a male or female. For almost all morphs, females will be more expensive by $50-150, depending on the morph. I've seen spiders between $150-300 and lessers between $300-600. Remember that you usually get what you pay for. Someone may be selling a lesser for $300, but it probably won't be the best looking example of a lesser; whereas if you buy a lesser for $500, it should be a lot better looking than the other.
3) 92 is perfect ^__^ and the humidity should be between 50-60% with 60-70% during shed.

I'm still quite new to BPs myself, so anyone can feel free to correct me on this information, especially on numbers one and two if you have more knowledge about this! The information here is merely what I've seen, heard, or observed since discovering the world of ball pythons =)

Good luck with your soon-to-be new BP!