TELUS currently charges $35 for unlocking phones. Unlock codes are available from 3rd parties for much less than that. I just unlocked my Dad's prepaid phone for $1.85 with a code from eBay. I understand there are customer support costs that aren't there for most 3rd party unlock services but TELUS should be able to do better than $35.
The $35 unlock fee can be added to the device balance for subsidized phones on contract and waived upon renewal of contract. This implies that renewing a contract gives you a free unlock on your previous phone which is a good incentive to renew.
The $35 unlock fee is offset by an account credit for prepaid phones if there is a minimum balance on the account.
Continue to charge the $35 unlock fee for other cases.