Adam asked how much a guild should charge for a spot in a ZA raid, but Lane on WoW LJ comes at the issue from the other side: how much should you offer? Personally, like a lot of comments on Adam's post, I'd offer nothing -- I'd much rather earn everything I wear rather than just paying out gold to be in someone else's raid run.
But certainly there are quite a few folks out there, who, for whatever reason, are either at their wits' end and ready to spend money, or just too lazy or busy to join a guild themselves. So what's the going rate for a boss? Adam's post landed pretty low -- 500g to 2500g definitely got the most votes, and I'm guessing that was mostly because there was no free option. So, depending on how well you know the guild you'll be running with, seems like a good idea would be to start at around 500g, and then head upwards from there.
Of course, especially on a relatively early dungeon like ZA, it's not really that hard to get 10 people together and run it every week -- even if you're not a hardcore raider, running just one raid instance a week is enough to get you learning how it works and building up your gear.