Yeah - the best Stratix 10 can do 9.2 TFLOPs. A GTX 1080 Ti can do 11.3 TFLOPs. Now I can't find any price info on those Stratixes, but given how expensive these FPGAs generally get, I highly doubt it will be anywhere near the $700 for a 1080 (and that's for the entire card!).
I am not saying these things don't have applications, but the "usual" computational workloads are not one of them.
FPGAs aren't known for their FLOPs though. Sure, the high end ones pack a punch, but compared to GPUs are still extremely expensive.