I've had quite a few clients and prospective clients call and ask, "What is a good conversion rate?" and "What is a good shopping cart abandonment rate?"
While Marketing Sherpa can give you the average conversion rate for your general industry, many web sites have intricacies that make them different from the "average" site. Additionally, many smaller web sites process e-commerce transactions through "canned" e-commerce systems (like Miva Merchant, Yahoo Merchant Services, osCommerce and others) and either can't or don't customize the checkout process.
My answer to the question of a "good" cart abandonment rate or conversion rate is to re-frame the question. Rather than asking what a conversion rate or cart abandonment rate should be, the question you should ask is "What can I do to my web site to improve the conversion rate and cart abandonment rate?"
I tell clients to use the Fireclick Index to get a general rule of what their e-commerce conversion rate should be, then compare it to their actual conversion rate. Is it close to "average," or is it not in line with the average for your industry?
Once you answer that question, the real question is how you improve your conversion rate. We answer the question of how a site can improve by performing a web site assessment, which is an exhaustive analysis using qualitative and quantitative data, including testing a site with real customers. This helps uncover issues with web sites that make them easier to use, improving conversion rates.
So rather than asking "What should my conversion rate be?" ask instead what you can do to improve it.