Easiest way to do that is use the attributes controller & set your "Prices & Weights" area to discount a set amount. For example, here's what I did once:
I've got tub truffles in a lot of different scents, and obviously UNscented ones are cheaper/easier to make. For a time, I offered the UNscented ones ONLY at half the price of the scented ones. So I had a drop-down list of scent attributes to pick from, and for the "UNscented" option, I changed the "+" sign to a "-" sign and manually entered ".50" (regular price of the truffles is $1.00). Doing this told the cart to subtract 50 cents from the price every time the UNscented option was selected.
So whatever your normal price is...just enter half that amount in the price box for THAT attribute in the attributes controller, and be sure you use a minus sign, not a plus.
Hope this helps!
Michelle B
The Soap Peddler - Homemade Goat milk soaps, Beer soaps, Coconut milk soaps, Cocoa butter tub truffles & more!