I beleive that for a TRUE open jaw - such as EWR-LAX and then PBI-EWR, they take half of the roundtrip fare for each leg, and add them with taxes, etc.

Example:

You wanted the above routing.
Current EWR-LAX roundtrip on CO is $400. They would add $200 to your ticket for that leg.

Current PBI-EWR roundtrip on CO is $200. They will add $100 to your fare.

Your total fare is now $300, plus taxes, fees, etc.