New Upper receiver

I bought a bushy M4 about 7 years ago, and at the time, I cheaped out, and didn't spend the little extra money to get a flattop. Now i would like to swap out to a flattop receiver. I'm looking at either a DD or VLTOR upper receiver, I just need to know who around here can do a quality install for me, and what kind of money we are talking labor wise? I was thinking about calling taylor gun exchange, but I wanted to ask first to see if anybody had any other recommendations.