The cost of a good condition FR64s is around US$1,500, at least it is over here in Asia. A couple of months ago, my friend did the whole AnalogMagik 64s to 66s conversion, tonearm rebuilt, upgrade bearing and internal wiring at roughly US$3,500. So yes, depends on the shipping cost, you can get an AnalogMagik converted FR66s at the US$5k range, maybe slightly higher.
We have compared the FR66s vs 64s, and even with a mid to high compliance cartridge, we preferred the FR66s. And considering the Kuzma Safir 9, with its 60g effective mass, has been successfully paired with many mid compliance cartridges, I wouldn’t worry about pairing the DS GrandMaster EX with the FR66s.
Actually, I have a DS W3 which I have not mounted yet, but I’ll certainly try it with the FR66s, FR64s and the Kuzma Safir when the time comes!