I agree with most of you; the conversion ratio (for your traffic etc) is an important element when choosing a sponsor. If I had to choose between a sponsor paying $10/signup with an 1:100 ratio or another one paying $45/signup and converting 1:1000, I'd definitely go for the first one...
(which is exactly what we are trying to find out at
http://www.sponsorreport.com - so submit your sponsors' statistics often in order to find out which ones are converting good and bad)