There was a formula around a couple years ago that guestimated how many banner clicks you should see from 1000 gallery surfers (as well as free site, pic post, top list, etc)
I wanna say it's 1:1000 that click on a banner/ad for a gallery page, which would be 90 hits for you 90K & if you got 1 signup, you're doing good
(above numbers are a complete right out of my ass guess)