As for my band, we just make our prices dirt cheap to make them move faster, thus more people buy due to them being cheap and affordable. We're still a local band... we can't charge $17 for a CD and expect people to buy it
, we charge $10 for t-shirts/girl shirts, $15 for long sleeve, $20 for hoodie, and $5 for CD's.....
Sadly our grand plan of "Sell Cheap = More Sells" didn't work to well on our tours, and we lost a lot more than we gained since no one cared to support us (and the lack of any turnout didn't help either).
I think it's all about tour cost and making sure you will have enough for gas/food/lodging, etc.... but when it comes to bigger tours, you obviously have to charge more to pay back the promoters (i'd guess), pay off your debt with the label, or match with a set price that the headliner puts on (like Six Feet Under....).
It's all about the business really, and not much about the music once you're signed. You're just a walking advertisement.