One two-region bucket would be cheaper than two one-region buckets. I’ve done analyses of similar systems and calculated the costs necessary.

You pay for durability and availability in the form of disk overhead, CPU, and network. Each encoding scheme has some expected cost. If your overhead for one-region is $X per gigabyte, then generally speaking, the overhead for two-region is going to be less than $2X—each region is more durable + available because of the copy stored in the other region.