From everything I’ve read, ECC memory is an orthogonal layer of protection to the FS-level stuff in ZFS and BTRFS. So the “if you care a lot about your data, use ECC RAM” mantra would seem to be as relevant to BTRFS as it is to ZFS; not required for either, but a good idea.
The cost premium of ECC RAM, in my recent experience, is minimal. But it does require a motherboard that supports it, which can add to cost, and decrease flexibility.