Is it possible to replace original vinyl windows with retrofits?
The price you mentioned may seem a little high, then again maybe not. Your question, if you can use replacement windows, would be no. The extrusions of the  vinyl windows would not allow that. Just to get it out there, you could use them, but only after you remove the vinyl window completely, and install them like it was new construction. To do it that way would be a little counterproductive.

Depending on your rough openings, they may have spec'd custom sized windows that sent the price higher. I just gave a quote for replacing windows in a 1940's home with double hung weighted windows for just under $1000 each in Maryland. I spec'd 3-0/5-6 Andersen double hung windows with screens for new construction. It would be a total gut of the original window so the weight pockets could be filled in and reframe the RO. I could have spec'd replacement windows here and quoted it a lot cheaper, but that would have meant a lot of old wood still existing. I did not want to have that since the homeowner was remodeling the whole house in stages.

Keshlam's comment about researching replacement parts is a good route to take too.