Your explanation is pretty cool since the interest rate is based on the value that is available in the wallet. Although I have seen some wallet retrieval services that collect a fee of 20% flat rate without negotiating on whether the amount on clients wallets can be huge or not.
I think crypto wallet retrieval services need to be flexible and relate with clients in a comprehensive way in case some might want to give their own rate which might still be adjustable. In a case when the wallet recovery procedures are simple and easy, slicing the price can attract more clients since service rate should not be fixed and less affordable.
Sometimes I just feel they should be at least considerate to some extent, because without the client approaching them, they would not have much job of recovering wallets, and for those reasons, it would be best to encourage the client as well because a job well done would have a good reference and referral for the organisation, which also helps in building portfolios and also fostering relationships amongst organisations and their client.