The way I've always done things with my friends is like this:
I buy some weed, I share it with anybody I consider a friend without asking for money or anything in return. Sure enough, next time they buy weed they will share it with me without asking for anything in return.
I have no idea if I break even, but I know it makes people really happy to get smoked out so I continue to do this. I've always considered weed to be one of those things you should be generous with. I never want to prevent a friend from being included just because they don't have 5 bucks. I get high either way, and one more head isn't going to break the bank.
The only exception to this rule is if some randos come up to me in parties and want to smoke my weed, I'll charge them 5 bucks to get on a blunt or something because I know I'll never see them again.
How about you guys? What are your opinions on weed sharing?