This includes:
1) Your landing page (does it convince visitors to buy, and if they are convinced is it easy to do so?)
2) Your ad copy (the more specific, the more targetted and better converting the traffic will be)
3) The Keywords (again, more specific is usually better, the more general keywords have more browsers; also things like "free" mean less likely to buy, although it doesn't mean they won't convert.)
Bottom line, there is never a set conversion rate, anywhere.
I guess misinterpreted the sentence on teh google which says :
Google AdWords ads connect you with new customers at the precise moment when they're looking for your products or services. The Google Network reaches more than 80% of Internet users.** Unduplicated reach of Google and Partner sites, according to Google analysis of comScore Media Metrix, May 2005
Sorry for that.
BTW, if I am not going off-topic, which program would u suggest for a first timer to go for, ADWORDS or merchant program like pay-per-sale?