Yes, the seal on 2 of my tall, skinny windows have failed. One is opposite side as yours (drivers side just in front of the door) and the other is the bedroom slideout. That one has filled with an inch of water! The driver's door glass is just fogged & growing moss.
Both windows are by Creation Windows, who are now out of business.
http://www.parkin-acc.com/ will/can supply replacement if the know the Creative Design part number, but the cost, as you pointed out, is outrageous.
Mine (& likely yours) consist of an outer metal shell. The inner/inside ring has screws that somehow sandwich the glass between. The Parkin rep, suggested removing the glass & taking it to a local glass shop to have it separated, cleaned, and resealed.
I contacted Scott Pullin (GS VP of Operations) himself who said that is about the only recourse. According to Parkin, they do NOT supply windows to Gulf Stream, as Creation Windows once did.
I verified the manufacturer & type of glass by a very close, careful examination of the window. You will find an ectching in a corner. Not only does it identify the manufacturer (Creation Windows), but there should be a couple of codes (AS2 is automotive safety glass rated for side glass. M2 or M6 apparently mean additional stuff, but I don't know what. Actually, to read them, I illuminated the etch with a flashlight and snapped a macro digital picture.
I'd be interested if someone could identify GS's newer manufacturer. I'm guessing they are making the same size windows.
Thanks for bringing up this topic... I'm also looking to solve the problem, and not for $500 per window! That's obscene.