Update our affiliate program

We have a built-in affiliate program, PHP, mySQL. It currently functions but we need some modifications. We can currently set the payout percentage. We need to have the payout percentage change as the revenue changes. i.e. all affils get paid 20% for the first $999 in sales. At $1,000 and more, the percentage goes to 25%. We also need some mods to the reporting section. We want the capability to search affiliate sales and calculate total sales and commissions by month. Alot of the program is already built so this should not be difficult for the right person.