As demonstrated [here](http://dustymabe.com/2013/09/22/btrfs-how-big-are-my-snapshots/), this is actually fairly simple to do.

First, enable btrfs quotas:

 # btrfs quota enable /btrfs_subvolume

And then run:

 # btrfs qgroup show /btrfs_subvolume
 OR
 # btrfs qgroup show -f /btrfs_subvolume

Which in Btrfs v3.18.2 shows you this:

 qgroupid rfer excl
 -------- ---- ----
 0/260 1.09GiB 1.09GiB

The 0/260 is the subvolume ID, and the excl is the exclusive data in the subvolume. If you delete the subvolume, that's how much space you'll free up.

Edit: According to this [link](https://github.com/openSUSE/snapper/issues/51), this appears to be the official recommended way to do this.