Dave,
The law is very specific as to what an FFL holder can charge to facilitate the sale between private individuals. They can charge $10 for the CBI background check fee and up to another $10 for facilitating the transfer. Charging anything above $20 for person to person transfers is not allowed under the current Colorado law (this is not to say that it is not being done).
C.R.S. § 18-12-112(2)(d) - A licensed gun dealer may charge a fee for services rendered pursuant to this section, which fee shall not exceed ten dollars.