Come up with some stats for how much traffic the "Average" gallery link will get? 
I buy a LOT of spots from the big guys all the way down to smaller niche tgp's in the 10k/day range ... and 90% of the time I am told that the seller has no idea how much traffic I will get.  I've pretty much stopped asking. 
Now I KNOW that the thumb / text link can have a HUGE effect on the number of clicks, but why not just give a range?  Like, 1-5 spots get 20 - 40k uniques.  Even a huge range like that would be better than nothing.  
BTW, I thought to post this because of 
this  thread.
That is all, just had to throw it out there. 
In closing, if you have gallery spots for sale, please post 'em in this thread ... even if you have "no idea" how many clicks I will get. 
Thanks, 
-Phil