$300 for a window is more reasonable than $300 for a faucet.

Installed, I paid $1000 for 4 windows.
$300 installed per window isn't entirely unreasonable for custom cut windows, depending on the size. Are you just doing hoppers?
If you can get replacement hoppers at HD or lowes that fit the existing openings, then I'd say do it yourself. It didn't look too incredibly hard. My problem was I couldn't buy windows even close, and it would have cost me alot more to enlarge the openings. Also, remember that the hoppers at Lowes are pretty crappy and IIRC are single paned.