I level my lockpicking along with my cooking and fishing and professions. It isn't hard really and I never even thought about tips for unlocking a box. Since I have a rogue, I've never asked anymore to do it. I think the price is low is because you don't really have to make an effort to level it, it's not a money sink like enchanting/engineering is at the beginning and you don't have to run instances over and over to get that pattern to drop.
Talking about tips, then mages and locks should also get tipped for port, teleporting, food, stones, summoning. But we don't. We don't ask either, cause it's just what we do. I've never been tipped once for any of those thing, although I will pay a mage to teleport me to Shatt for the first time.
Perhaps we pay enchanters because we perceive that the enchant is of a certain value to us and opening a locked box has very little value in comparison. Or the enchanters guild has a monopoly going and anyone who doens't cough up a tip, won't get an enchant.
Anyway those are my thoughts and humor on the matter.