You should only need an additional three training items:
- Ground reference maneuvers
- Night requirements (3 hrs)
- Instrument requirements (3)
You can cover the FAA night requirements pretty much at the same time as a JAA/EASA night qualification.
Your FAA ground school is probably not fantastic so count on an additional 5-8 hrs of ground instruction to get you ready to pass an FAA oral.
The FAA written test is $150 and the examiner fee is probably around $350.
So depending on which airplane you fly for around $2,000 extra you have both.
Go for it!