I just installed a basic Pella 6' slider (just under $300 at Lowe's) which replaced a large window, but I had the room gutted and am residing the outside, so it was no big deal.
I have to take exception to what Mark said (good natured, of course, Mark!). I agree with it in part. Yes, it will be easier to BUY a standard replacement door, but I don't see how it could possibly be easier when it comes to installation, if resizing the rough opening is required (which it would be)
A local vinyl window company may be alble to offer a solution that would be minimally invasive with regards to tearing out the old one.