custom Atribute equals a product price

[Dezaine studio]

I'm setting up a store, but i have one issue with prices, if the user wants to order a sample, the price will be diferent for each product sample.
I want to set a fixed price for each sample, for example the product costs 50 euros, if you choose to order a sample the price would be 22,5 euros.

I can't get this to work, i've tried using the "equal" sign (=) before the price, but the system only let's me use + or - .

Is there a way to make this work? in the previous version of virtuemart i used the "=" sign and everythin worked great ...

Thank you ; )