I believe your issue is something else entirely. With the method of setting price using variable and expression for some reason it is not reading {ORDER TAG PRICE} I read where @QMcKay has already brought it to @emre attention in a different post.
{ORDER TAG PRICE} should have shown the value of the price in ORDER TAG If you set static prices for ORDER TAGs it works fine.
I have not built a Discount solution yet. I plan to soon.