A fee per kilogram placed in orbit, refundable if the entire mass is deorbited as a single object, seems like a reasonable starting point.

If that fee is the entire estimated near-term cost to recover the object, it would also create incentives for entities putting large amounts of mass in orbit to focus on improved strategies for removal: if removal becomes lower cost than the fee, it's worth paying someone to recover your debris if a satellite fails in order to recover the fee.