Bug In Pricing ?

Hi,
I've noticed this strange behavior while checking an item cost (while buying as myself):

I've setup my markups as follows:
12"x 15
14"x 21
16"x 34
20"x 58
24"x 88
and so on

Up to a 16"x print I see the correct markup removed under the image.
When I switch to a 20" and above I get totally screwed up values (I'd expect a 58$ value but I get 63$ and so on)
If i change the 20" price and get back to the product page it changes to the new value...plus some extra $.
As I increase the size this amount is getting bigger....it's driving me nuts!

Where I'm doing wrong ?

Can someone please do a test on its own products and report back ?

THANKS !