You can't repair the glass itself, you need to put in a new one. Many window manufacturers have 10 or 15 year warranties on the glass. I recently replaced 2 fixed upper glass's, 1 sliding lower, and 1 glued-in bathroom fixed window.
For your sliding window, removing the upper glass is fairly straight forward, (normally) for this glass section you remove the trim (called glazing) from the inside with a putty knife. It's a little bit annoying but it comes out eventually once you figure out how it works. On the "outside" facing part of the glass there's foam tape (mine was only 1 side sticky) which creates the seal to the window vinyl frame, so with the glazing removed you literally just pull out the glass (or use a friend to push from the outside). [Note: surprisingly, your window looks like the glazing is on the outside and not the inside, but I'm not an expert on this and if you just need to work in reverse].
On the sliding part of the window, they normally just send you the entire assembly and you unclip and tilt out the old one and clip in the new one.
Also, on fixed windows (where you don't have an upper and lower part, where it's just one glass), those are sometimes glued in, which really ***** because then you need to cut the glue from the outside.
$900 looks like they're totally hosing you, trying to take advantage of you. On the inside of the window frame or inside of the sliding glass portion, look around and see if you find any labels. That'll be the label you need to take a picture of if you decide to file a claim with the glass manufacturer to send you a free replacement.
