Fee formulas

Hi, is there a fee formula that will calculate a different price for a certain number of items?

For example, I have 10 t-shirts that cost €10 each. But if the user enters 11 or more, the t-shirts are €8 each.

This formula doesn't work:

([FIELD_VALUE] > 10) ? [FIELD_VALUE]*8 : [FIELD_VALUE]*10


What formula would I use here?

Thank you.