I'm not sure, but here is the actual wording in the law.
"A licensed gun dealer may charge a fee for services rendered pursuant to this section, which fee shall not exceed ten dollars."
My personal interpretation of that is that they are not allowed, by the law, to charge you more than ten dollars for their service. Plus, obviously, the mandatory $10 CBI fee that you would pay on any background check done in Colorado for any firearm purchase.
If a dealer wants to charge me more than $20 ($10 for the transfer and $10 for the CBI fee) I go elsewhere. And, in my mind, they are in violation of state law by charging me more. Probably why most of them just don't mess with it.