Hello,
You may know of my company. If not, many 3dCart clients do. We are a marketing partner with 3Dcart.
http://www.3dcart.com/beyond-roi-shopping-cart.htm
As part of our partnership, we have agreed to provide 3dCart with our insights for their clients. We specialize in performance based marketing so I can most likely point you in the right direction.
On your question with Conversion Rates, you are not alone for the most part. Google is 27% higher CPC then Yahoo! usually and also Yahoo! tends to generate a more favorable "buyer" demographic. I certainly hope that you are utilizing MSN / Bing Ads.. This has show the highest ROI out of the paid search campaigns. Again, not as many conversions but very profitable.
Because buyers are becoming more frugal, we are also seeing better returns from CSE's (comparative shopping engines). All of this depends on your individual products, profit margins, selling points and the competitiveness of your pricing.
If you have some free time, give me a call at 954-374-2003 and I will have an evaluation put together for you as well as recommendations.