Maybe $3 as a block bet, and fold to this big move if it still comes.
We have bet flop and turn as though we are protecting against a FD. We actually have top pair, along with the second nut flush draw. Maybe villain has a T, in which case his flop call was a mistake, maybe he has something good, maybe he has the nut FD. Whatever. The flush doesn't come, and suddenly we don't bet the river, and it looks like we have missed the flush. He makes this big move, and we give it up. This feels a bit soft to me.