It's very hard to find cheaper 4K ones - I got two 28", 4K monitors for under £400 but it would cost double as much to get equivalent curved ones.
I recently bought a pretty nice Samsung 32" 4k monitor for our son - £269 on Amazon's Prime Day, so it would have been £538 for a pair. They had 28" ones for less, so a pair of those couldn't have been much more than £400. The thing that makes monitors expensive these days is not curving them. Its making them gamer friendly.