The 'you break it you pay for repairs' rule is pretty much universal. I personally will rent out my loaners for the cost of said renter lugging my gear up the hill to our field and back down again. Other members of my group usually charge $10-15. A couple of them will loan for free. And that almost always includes a hicap full of ammo as well.
In reading the other posts in this thread I'm amazed at how much some people are charging. Do people charge $50 and actually get takers? Four games at that price will buy you an Aftermath gun, six will get you a JG. I'd rather sit out airsoft for a month and a half to own my gun then rent at that price.