I have no clue what size or type window the OP is after but the quotes are probably jacked with a cripple.... pun. Pella is trash and over rated.
Marvin is excellent. Call a rep and give them the rough opening dims. If its really an oddity, push to see if they have other odd sized or something close in a warehouse. There's ways to make them work, even if it involves opening up a bit or downsize and re-trim. Get a contractor and provide them with your research and get that sucker for wholesale.
I've done new Marvin sliders doors, enlarged the opening - requiring siding strip, new header, misc. electric, re-trim, re-install siding + trim for $1200.... out the door (pun
. Good night.
BTW: Ask the Marvin rep for display showroom samples. Take 75% off.