I don't think there is a set price. Some people sell those little zip submits for $1.00 a piece. I guess for some people it is pretty profitable. I'm selling products with an average order of about $100.00 and my conversions are still pretty decent.
I think if you are targeting people who are looking to buy then you'll convert. For example, if you can sell a treadmill for $200.00 and can effectively market to that group, then you should still have a decent conversion rating. Granted, a lot of people are much more hesitant about buying high priced items online due to the risk involved of getting scammed. So just think about what type of products you are offering, I don't think it matters on the price too much if you can effectively market the product.