Are you talking about glass or the whole thing? Also, what (approximately) is the size of your window?
Are you planning on a specific type of glass for the french doors? Inside french doors don't require as thick of a pane.
Also as a suggestion: Check with some of the window and door replacement companies/construction companies. Some take the old ones out and dispose of them for the owner, and some companies make the owners handle gettin rid of the old ones.