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You also have to take into consideration what type of service it is. People are less likely to buy into something that requires a monthly recurring charge, etc.
Too many variables to make a plain statement like that.
I would expect a book at Amazon.com to promote far higher than a $5000 necklace from a full price jewelry merchant.
I would expect a well known name like Amazon.com to convert much better than a virtually unknown merchant.
I would expect a merchant with free or cheap shipping to convert better than a merchant with tiered shipping ranging from $8.95 to $24.95.
I would expect a merchant with a streamlined checkout process to convert better than a merchant with a confusing shopping cart and checkout process.
I would expect a merchant with discount prices to convert better than a merchant will full retail prices.
I would expect well targeted traffic from PPC to convert much better than general traffic on an untargeted site.
Some merchants share conversion information. I've seen some that show that their average conversion ratio is in the 3-4% range but that their top affiliates have conversion ratios in the 30% range. I'm sure they have others with conversion ratios under 1%. A lot of it depends on how qualified the affiliate traffic is and how well the affiliate presells the product.
Of course, this varies by industry, merchants, and affiliates.