1. Add simple product and keep its base price default price "0(zero)". Add other fields and proceed.
Goto Custome Option tab, add your custom option (i.e) add title "Size", select radio button for Input type .
Add Option size (i.e. 14, 18) and price 45, 55 respectively and save your configuration.
2. After the process install module from here.
Download setup and paste all file in your magento setup. Goto adminpanel and all clear cache.
Goto Menu >> Product Options >> Manage Optioned Products, here there would be product list which would show added custom products.
Edit product and select option (i.e. 14) which you want to display on front end as default option, then save configuration.
Let us know if you face any query/concern regarding this.
Query solved? Accept as Solution.Thanks
Eric Baily