Depending on what Gorell series window they were offering you, the $5000 quote may be just fine. For example, Gorell 5300's would be just fine at that price, and if that was with triple glazing/krypton then even better.
If they quoted the 5100, then I would keep looking. There are a lot of better windows than the 5100 you could get for $5000.
The Lang window is not horrible, but it is not up to date on glass technology. They still use metal tube spacers in the glass and spiral counterbalances are standard, though you can order them with constant force if you specify. They will make you just about anything of any size. Just be aware that oversize or brown windows come with no warranty at all. This window is also not worth $5000 in my opinion. Again, there are better windows for that amount.