I'd imagine that the big variable on 8-in floppy cost is whether or not any modern use requires a magnetic medium of coercivity close enough to be compatible: the actually-floppy floppies are pretty simple on the inside, just a casing made of die-cut plastic sheet, some anti-dust pads on the inside, and the 'donut' of magnetic medium. If you have to commission bespoke magnetic medium because all the modern stuff is too high coercivity to suit high density magnetic recording, that could get unpleasant. If there is some user of magnetic film of appropriate coercivity, getting appropriately shaped pieces of it punched out of sheetstock shouldn't be too difficult.
Even if (and it wouldn't be too surprising) the floppies or their drives are ultimately impractical to keep running, it still might be easier and less risky to build a floppy drive emulator that speaks the appropriate protocol but uses some more modern storage mechanism. That's an entire cottage industry in hobby retrocomputing, allowing you to replace scarce oddball HDDs and weirdo floppies with CF or SD cards; and given the relative simplicity of historical floppy drive interfaces I would strongly suspect that you could get an all-American engineering team to cook up a drop-in replacement without too much trouble.
That said, hardware is certainly the area where obsolescence is likely to become a real logistical problem first; so any attempts at modernization(incremental or wholesale forklift-replacement) should be aimed at trying to decouple the system from specific hardware as much as possible(the 'baseline' hardware profiles used by virtualization systems to accommodate guest OSes that aren't virtualization aware and capable of playing nice with virtualized devices, say, are already obsolete hardware; but will probably be just as available decades from now as they are today); but even where hardware is involved, the difficulty of replacing the system as a whole makes trying to incrementally replace the hardware(with well defined compartmentalization at various interfaces to make the future replacement of your replacements easier) a viable consideration.