Selling by the square foot?

Okay... I've gone through a few pages of forum topics, and didn't see it - I've also been going over the manual some...

Basically, I want to start out the purchaser with a size print, and then let them know how much other sizes cost, preferably with a real number, not a "+ $xx" statement - is this possible?

Right now I'm trying to make sense of the "offsets" stuff... And it doesn't seem to want to work.

I don't want to do separate products for each size. There'd be too many.

Clarification:

Right now, my base price is set at $3.00 per square foot. For a 2x3 print, the I'm doing a factor of 6, and an offset of -1. Which gives me a correct $18 price and all that, but people see a price of +$15 in the dropdown box - Is there a way that I can just have the sizes in the dropdown box? Or have it show $18 instead of the $15?

I know it's somewhere... frustrating...

Er... Offset of 1...