Spaces could be very useful for frequent / large backups. However it would be too costly to pay the bandwidth from Droplets to Spaces.
AWS never bills the bandwidth from EC2 to S3 (or vice versa). You should do the same: I suggest that you completely remove billing of bandwidth from droplets to spaces (and vice versa) when they are located in the same datacenter.
It could be even more interesting (e.g. for backups) if you could remove billing even for traffic between Droplets and Spaces in different datacenters.