This idea has been on the WHMCS forums for some time, its a great one, I approached 2 devs about making a plugin that used a different paypal account based on the transaction value for full licence i.e. not released for general use I was quoted $500 and $899, if they could release it they agreed to 25% and first year free.
so if your going to do this I am in, I know you have not asked for feedback specifically but I would be happy to pay around $300 p/year for something like this if it comes with support and updates.
Edit: for clarity this is still on my ever growing list of things to get done, I will be doing it anyway but would much rather my money goes in your pocket as I know that you know exactly how this needs to work so the chances of any issues are reduced.