Several ways to do this ...
Give the Product a base Price such as: 10.00
Then if 12" is $10 no change on price
If 18" is $12 then on the Attribute set the:
Price Prefix to: +
Price to 2.00
This will show as:
12"
13"
14"
15"
16"
17"
18" +$2.00
19"
etc.



