Add cost price modifier to option value.

This allows users to write reports which are based upon the increased
price of an option type vs. the increased cost to the store owner.

![option-value-cost](https://f.cloud.github.com/assets/764390/502699/6dc4cc30-bcb7-11e2-9b68-2db1e865ced6.png)

The functionality isn't used anywhere else, but we have a couple custom reports based on cost price that we want to use in conjunction with flexi-variants, and I figured others may find this useful.