I have a current quote for some Marvin replacement windows. The quote is for Integrity and Infinity; same sizes for each (suspect stock sizes, actually).
Anyway, I'm baffled given the minor construction differences, why the Infinity is 2x - 3x the price of the Integrity for windows, and 2.5x the price for doors. The salesman expressed a bit of a surprise as well when he was summarizing the costs.
I'll give you some examples (Cdn pricing):
38.5W x 52 1/4H fixed upper/awning lower $684 vs $1722
45W x 25H fixed $388 vs $802
72W x 82.5H sliding patio $1217 vs $3321
The Marvin fiberglass lines appeal to me, and I'm likely going to go this way, but there doesn't seem to be a rationale for the wide spread in costs.