I take it that by OPTION A you actually mean Product A

Option name: Size
Option Values: Small XL

A price=0.75

Add the Size option (both values) but for the XL put 1.05 in the price field in the Attributes Controller.

B price=0.60; XL attribute price:0.40

You may have to change these amounts depending on whether or not you're displaying prices with taxes.

Experiment - you're not going to do any (permanent) damage, and you'll learn a lot!