I would love to have the option to specify price adjustments not as prices added to or subtracted from the base price, but as multipliers or percentages of the base price. I see a few places where this could be useful:
-You have an attribute with differently-priced options for a product, maybe the size of a product. You want the price of the larger size to be 50% more than the price of the regular size, but the price of whatever you're selling fluctuates frequently. This way, you only have to edit the base price when it goes up or down, and the price for the large size will adjust accordingly. Without multipliers, you have to edit the price adjustment for each option.
-Similarly, you have a product kit where you want to give the customer a 25% discount over buying the component products separately, but the component products have attribute options which alter their prices. You simply cannot do this with a flat price adjustment.
I imagine the implementation would have the store administrator specify what type of price modifier they want: the current standard price adjustment (-2.00 for an $8.00 base price), a percent multiplier (75%), or a percent adjustment (-25%). And in these cases, they could choose whether to display the modifier in the attribute selection process as a (calculated) flat price adjustment, a multiplier, a percent adjustment (formatted as "save 25%" or "25% off"), or the final adjusted price. This would apply similarly to the product kit module.