Hey there,
I track all my upvote purchases, here is what that vote cost me and what it earned:

On June 21, 2019 I bought a vote from @tipu, I sent 5 SBD, I received an upvote for $7.070 on Steem. Since 75% of the vote goes to the creator, I get $5.303 from that upvote, plus curation reward benefits of $0.05, equates to 7.05% profit on my initial investment of 5 SBD.