Wiki Fees for just Coding?

[BFAITHFUL]

I work for a billing agency and we now have a client that only wants us to do the coding & not the billing aspect of it..... all of our current clients we are doing everything including the billing side of it & so our fees are based on a percentage of what they bring in....

I really don't know what pricing I should give them if we are just to do the coding ..??
Any ideas? (eg. a fee for each op report or based on timing?)

Thanks