I am new to zenfolio as well and having the same issue. What I plan to do is, or hope to be able to do, is setting up different price list with different formula (since a formula is global to a price list) and based on different print size/products. The thinking is that I'd like to apply higher markup for larger prints and unique products.
If I can add more than one price list to a gallery, then clients can be switched to different price list depending on which product they want *for the same image*.
Right now as it stands, I would think the only way to do this is manually calculate and apply price for different products, which is very time consuming and not easy to change, vs. the formula approach.
I am going to search the "new feature request" section and if there is not already one. I think this is a strong case for consideration, at least for me.