Failing DMFs can last for quite some time, and might never fail in any kind of major way.
Depending on the style fitted, failure can range from losing drive, going so off balance it’ll cost you more in dentist bills getting fillings put back in than replacing the DMF, snapping crankshafts (this is also an issue if you fit a solid conversion), wiping out the gearbox input shaft, or the complete catastrophic failure where large chunks of metal potentially wipe out anything in the DMF’s path as it ejects itself through the gearbox casing (this was a pretty common Landrover failure method, and why early TD5s got recalled for new flywheels).
The dealer will be expensive because they’ll likely be quoting for genuine parts, which will be expensive. If you’re happy to use the dealer, then it’s always worth asking if they’ll quote for aftermarket parts instead.