onchange is what Ray has put in place for us Jason so we're sticking with it because it cost us $198 in a Premier Support ticket! :(
Regardless though, adding a calculate price button seems, to me, to be adding another level of complication/choice/click for the customer. They want to see the price and add to basket, not to have to calculate it themselves, no?
Do you feel that my solution, as described above, will cause problems?