I will be honest, both are not cheap. It's much easier to purchace an aftermarket headunit with remote. I have sold that job for as little as 250.00, up to 700.00, but you will spend 2-6 hundred on a good aftermarket head unit. To tap into your OEM head unit is a different animal. That job starts at 500.00.
Price really is affected by what it actually takes to complete the job.
Here's a link to a similiar project were I modified an Alpine CVA-1005 to use the OEM Steering wheel controls and OEM headunit buttons in a BMW. The head unit was completely re-designed to fit the dash of the BMW.
http://www.sounddomain.com/id/chris_hill
It is expensive to do these types of modifications, this is why I always suggest trying to find a wheel that will work.