It sounds like that price was for a custom made window to your exact dimensions.
Although it might require a bit of trimming of the existing opening, if you shop the used and surplus RV parts places you may be able to locate a back window from a motorhome that was designed as an escape
hatch. I'd even put the search on eBay where you will find a number of RV surplus places advertising stuff that you can send a request to for what you want.
And, if you are ever down in the Elkhart, IN area, there a number of dealers down that way with surplus RV parts.