Shopping cart items, including adoption groups, are not configurable to have variable pricing today. However, we do have this on our customer request list as a potential future feature in Shelterluv.

What some users opt to do today is set the prices on variable items to be the highest possible price and discount them down as needed. You'd want to be sure to configure a discount reason in Configurations > Shopping Cart > Transaction Discounts (possibly something like "Variable Pricing") so that your accounting team is aware of why the discount was applied.