PHP Crediting Script for Freebie/Referral Site

I own a freebie site where users complete offers to get prizes currently i have a script that only allows 1 offer to be completed then their account is credited (thru a closed loop URL with the affiliate) i would like to add a script that allows me to change the weight of certain offers. for example: 1/4 of a credit, 1/2 and so on. i don't want to spend more then $50 for this, as i know its a relatively easy concept but i just cant figure it out if you need more information, p/m me. thanks jared