Price Differential from base price

I need to have a Characteristic variant on a size that is incremental. For example the base price is 9.00 but the size 2XL is 1.50 more or 10.50, the 3XL is 3.00 more, etc. It's the same price differential regardless of the price (always 1.50, 3.00 4.50 or 6.00 more). How do I do this?