Your question is roughly quantifiable. Send an email to the guys at GTS and ask them how many clicks your spot will get for a given spot. They'll give you a rough estimate, and I would more or less trust these guys to give an accurate estimate. They should be able to give you a much more accurate estimate of the unique visitors that the TGP/MGP's themselves get--as they really can't say for sure how many will click on YOUR link. But let's say that the GTS ad you're looking at gets 700K uniques/day.
But, I would also look at your own gallery submission history to get good idea of what kind of clicks you can expect. Again this is very rough estimate. For example, if you submit to a gallery that gets 100k uniques per day, and while your gallery link is towards the the top you get 8k unique hits to your gallery page per day. You can roughly say that you'll get 8% of the total uniques during your top spot period.
Thus, you can double check GTS's estimate of clicks by taking their total uniques and multiplying it by 8% (or whatever the # is for YOU).
Now onto phase two. Of the people that normally visit your gallery pages, how many actually click through to your tour? (The button that typically says "Click here to see more" etc). Let's say it's 5% of the total visitors to the page actually click through to your tour (or affiliate's tour). Now, of those 5%, how many actually buy something? Let's say, after all this filtering, your final conversions are actually really good, like 1:10 or 10%. Let's say you get $20 for every sale (let's ignore the possibility of recurring transactions here to simplify).
So you basically take:
[Total GTS uniques]x[% clicks to gallery]x[% clicks to tour]x[% conversions to sales]x[dollar amount of a sale]=Total dollars from the ad space.
Or in our example:
700K x (0.08) x (0.05) x (0.10) x ($20) = $5600.00 (total revenue from ad)
In this case, yes, buy the ad.
You're going to need ad tracking software to be able to track and estimate this.
I can't be held responsible if you fuck this up. Or if I'm totally wrong to begin with.
Anybody else?