Samsung Galaxy S6 is $540 on Amazon. There might be better deals. And it also has other uses.
I agree with your general point though. I am quite happy with my $170 Motorola smartphone. I would consider this VR attachment if I already had a compatible phone. As it stands, I will probably wait for Oculus Rift V2. Facebook intends to price them competitively, and there's a chance that second consumer version will be usable without too much headache.