When you sell your upvote, you do not have to send any SBD to MB. They just use your voting power.
Sounds like you are referring to "Buying votes" this is when you send in an amount of SB and they up vote your content. The lowest you can send in is .01 and you normally get an up vote worth .027
Hope this helps. Check out part on on the series it explains that.
It does help..
Thanks friend