Thanks for all the replies.

The quote was for just replacing the glass, not the window.

We looked at the structure together and figured out that there are
four wood pieces screwed down on the outside that need removing to
remove the glass unit. Shouldn't be too bad.

The weird thing is that over the phone they told me it would be about
$400 for the glass (I gave them approx. dimensions within an inch or
so) and $200 for install. That sounded fine. After they came out,
the glass went up to $500 and the install to $600. (It was a
message...I haven't asked them what happened yet).

Ok, sounds like it's not a DIY. There's (at least) one other shop in
town that does this. I called them too and they gave me a similar
price over the phone. I'll see what they say when they come out.