1-2% it is the norm for tgp gallery traffic
you can do 2-4% - it depends on how much you qualify your traffic.
If you tell surfers the site is free to join expect a higher click thru rate than if you say it's $2.99 for 3 days etc.
You will get a higher CTR rate but shittier conversions and vice versa usually.
And yes because each sponsor counts their clicks differently - for tgp traffic a lot of people look at how many uniques per sale.
Say 1 sale for every 50,000 visitors etc. - that is pretty average.
BTW - in case anyone is wondering SE traffic sent to tgp galleries will get you a 25% CTR
