It ultimately depends on your traffic quality, amount of visitors, etc. I guess this applies to all sites regardless of the niche.
I've got some deals with ad networks because I run an adult-based image hosting, not the regular all-in-one image hosting, so they pay really well for CPA, not-too-annoying in-page push that covers all my hosting expenses and there's left over for me to enjoy a nice dinner.
Now, this wasn't always the case, for the 8-9 months everything came out of pocket, and now that a few (hand full) of forums use us as a primary option, as well as few private individuals that use us for their sites, we are serving around 30M requests/day, around 9 to 12 TB bandwidth/day. So things got nice really fast.