It really depends on the TGP, the quality of your galleries and what niche you are into....
I think a top spot at
www.thehun.com costs about $300 and the traffic is generally good for more sites.... but you must consider the bandwidth cost too....
If I was you I would look into some smaller TGP's that deal specifically with your niche... the convertions should be a shit load better....
Hope that helps...
If you can post an example gallery I would be glad to offer some suggestions :-)
Cheers
Terry
www.voyeurzine.com