Hi Max,
A good idea would be to leave the exact amount out of the contract and specify that the fee depends on the fee the RIPE NCC charges you. That way you won't get into trouble when the fees change later.
It is a VERY BAD idea. None of healthly minded financial people sign that ;)
Why? You get charged for the resources that those end users have. It is not strange to charge them for that amount. There is no way for them to keep those resources without paying the amount set in the charging scheme by the RIPE NCC General Meeting. The same as an LIR who has to pay the RIPE NCC membership fees set in the same charging scheme. You as an LIR have signed a contract with the NCC without knowing the exact amount that you will be charged next year. Why would an end user be any different? Something like "The yearly fee for Provider Independent resources is determined by the RIPE NCC Charging Scheme. We will send you a bill for the amount specified in that Charging Scheme". Your lawyers should be able to put that in some acceptable language for in a contract :) - Sander