There are quite a few discussions ongoing about the price of an FP2 and variants. The main issue is not the absolute high-tech hardware but mainly the low number of devices which are built. Well, and the modular design surely also plays a role, but is one main factor in the aim for sustainability.
The two following forum threads om to my mind that might be interesting in this regard:
https://forum.fairphone.com/t/unofficial-second-hand-fairphone-2-marketplace-sell-buy/16305