A:
I don't know what Best Buy would charge, but my wife and I installed our over the range microwave, which I assume is what you are asking about.
If so, the question in your case is if the existing brace will work and if the holes will line up (there are 3 holes in the upper cabinet, two for supporting screws and one for the power cord). If not, then it's a question of possibly removing the existing brace and installing the new one. Then you may have to drill new holes. The brace, hardware, and Instructions were included, but we supplied our own tools. (level, screwdriver, drill, and bits). We both have experience along these lines and please note, lifting the unit into position is a two person job
Hope this helps, Best of Luck!
LocalBoy