Depends on your game genre but I suspect generally lower on portals, unless the portal sells it very cheap ($6.99) so that help increase the CR halving revenues.
When you look at your typical conversion rate, is it generally lower or higher on portals (vs. on your own site)?
I would also expect it to be lower on portals, some of that has to do with how people would find your own site. Many are looking for the game when they find your site. That is a very different customer group than one that is just looking at a list of games when they find yours. The first group is more invested already and thereby the CR would be expected to be quite a bit higher.
We see a higher conversion ratio on the game specific sites that we do for each of our games, but conversion ratio and sales volume are two different things.
Generally, conversion rates are far higher for traffic that comes directly to your site vs partners' traffic, since those who come direct to you are more likely to be looking specifically for your site/products.
Similarly, if you bid on the keyword for your brand name and a more general industry keyword, you'll find conversion is far higher for those users who click on your brand name, for the same reason.
Dan C. Engel, CEO, FastSpring.com
Email: dan at fastspring.com
Product Demo: http://www.fastspring.com/