Replacement windows...is this worth it?

This May we are having replacement windows installed in our home. The company that's doing our 10 double-hung windows gave us a quote today on replacing our large front floor-to-ceiling bay window. (This window, by the way, is original - it was built with the house in the '40's.) $1,700 for 2 panes of tempered gl, new muttons and framework, and aluminum capping on the outside. My question pertains to something another company we received a quote from told us: that we'd need to have the entire thing ripped out and re-installed (i.e. - new construction). Their estimate, of course, was more than double the one we got today. What's the best way to go? Can this type of window be replaced, or do we really need to save up for an entirely new structure? We don't plan on living in this house for the next 30 years, but I would like for the job to be done right. Thanks.