I used to work as a sales rep for a carpet manufacturer. There are a lot of different factors in pricing carpet. Nylon, for instance, will cost more mainly because the raw materials are more expensive, but holds up the best of the man-made fibers.
We did our whole house which will be cheaper to do it that way (instead of room by room) because of the way you have to order carpet. Don't ask me to explain, cause I don't get it fully.
Many installers will have a minimum labor charge (I know the ones that Home Despot use do), which is only a factor with small jobs like one room. Some retailers may play games with numbers. There's one here locally that I went to for a competitive shop once. They gave me this quote but said that if I went for more carpet than what I needed the total would be less. I don't know if they keep the waste and sell it as a remnant or what, but it didn't make sense to me.
Mystery Machine, was that price carpet only, or did that include pad & installation. If carpet only, sounds like you got top of the line.
I thought that it was going to cost us $300 or $400 to recarpet our family room. The room is about 16 x20. It ended up over 1000.
That's because, no matter what, you're going to have a seam and some waste. Carpet comes in either 12' and 15' widths, and some plushes only come in 12'. There are some in 13.5' width but not many. Even though you only need 320 sf, you will need anywhere from 384 sf to 480 sf to avoid a T-seam.
Not only do the get you for the carpeting, but if you want anything more than "standard" (read, "crappy") padding, that's gonna run you even more.
True, but higher grade padding can increase the life of the carpet. I don't recommend anything less than 6 lb rebond for cut pile and 7 or 8 lb high density rebond for berber. If someplace is advertising free padding, check the quality of the free padding. That's one good thing I can say about HD -- their standard pad is the 6 lb pad, which is the upgrade at other places.