How about you can pay 500$ to increase/decrease by 5lbs one player, once a year?
I could agree with paying for an increase...but 2 years worth of money for 5 lbs?
I think that you should be able to buy in 5 or 10 lb increases but limit it to 25-30 lbs total. Also limit it to one purchase per year but the cost should be something that someone could afford year after year. And still have mony left for potential checks or contract extensions.