I should add my experience.
I went to Lowes and Home Deopt. Different but similar prices.
I went to the manufacture site, looked for dealers in my area and I emailed them all (about 20-25 of them) a list of my window sizes and options.
The price differences were HUGE. They are all buying from the same company/location. So some of these guys are making a killing with their markups.
For example, one wanted 13k (no installation) and the lowest was 6,700. Again, all buy from the same company, so the middle man is the one making the killing.
Also, make sure whom ever you go with, have them do the final measuring
For me, it was $35 bucks and well worth it since any problems means they will correct it at their penny.