The charges vary depending on Domain Name type and transfer type
It would help if the OP advised as their current scale of charges (to customers)
.com .net .org attract transfer IN costs, which you will be paying
You should therefore be passing AT LEAST this cost on to customers
If the customers are purchasing other services then discounting the transfer
of these Domain Names is possible, but going below the cost price will again reduce the profitability
If customers are transferring out you MAY charge an admin fee, but this may put off customers transferring into your services in the first place (and if the cost is hidden may result in negative reviews)
As above, .uk transfers are (currently) FREE