The book says of Consume Wisp:

Costs either an Amethyst Shard if the wisp is owned by you or has allowed transfer with the caster or caster's owner, or half again as much media as the consumed wisp contains if the wisp is owned by another and has now allowed it.

However, when I cast it on a wisp I own, it seems to consume (with default configs) 1.5x the amount of media in the wisp, as if I didn't own it.

I took at look at the source and I think the cause might be that the fightConsume method in BaseCastingWisp has its logic backwards; ie. it looks like it returns true when both wisps have the same caster.

+1'ing this, I wrote a wisp spell to, when the caster is low on media, trigger a spell circle to spawn a fodder wisp that links with the primary wisp & allows transfer, with the primary wisp then eating the fodder wisp. Both allow transfer between each other, but the primary wisp still pays the full price rather than 5 media.

Edit:
My bad, just checked the source & saw that the issue's been patched & the patch just isn't released yet.

This shouldd be fixed in the latest release, please make a new issue if I failed to correctly do that