The FNC has not been imported since '89 due to the ban by George H. W. Bush and it can serve as a host to a registered sear. With the sears running between $3,000 and $3,500 and the conversion running around $500 to make a Class III rifle that goes between $7,000 and $8,000, it's no mystery why people charge what they do for the FNC.
The FNC is a really nice platform. I like it a lot. It opens up like an AR to reveal an AK-style gas piston system and the whole rifle is more rugged than the AR. Bang for buck, the FNC is one of the best deals you can find in the Class III market- $7500 for a proven assault rifle that is chambered in 5.56, and accepts AR mags is really pretty good when you see people paying over $10K for AR's, AK's, and HK's.
The one thing that may present a significant problem for the FNC owners in the future is the availability of spare parts but I suspect where there's a need some enterprising individual or company will step in to fill the void and produce the parts. I know DSA produces spare firing pins. I can't remember who makes it but I know there are some accessory railed handguards made for it as well for those who want to add the accessories (lights, lasers, VFG, etc.).