Ouch, I hear stuff like that all the time though. Fucking vultures. My partner and I have a window/door install company, and we have people buy their windows directly from the manufacturer per our measurements and then pay us for the install after it's done. We make more per window than any of those companies will ever pay installers, and the people that hire us save like 40% of what they would've paid those scammers. It's bananas, sometimes we have to convince people that we're not scammers because they don't understand how we can be that much lower than "the industry standard".

I'm sorry you're dealing with that, most window places are an absolute ripoff. This should cost less than $500 all in--materials and labor.

Try calling up a building materials store that sells windows and ask them for a recommendation for an installer for their windows--better chance to talk to someone that knows what they're doing.

Order a replacement window-they don't have any flange on them and can be made to size. Rent or borrow a trim brake--then some white aluminum trim coil to cover the gap and make a sill angle. You don't have to have wood all the way around, just behind the screwholes.