You'll spend 4-7 Steem to get 100 SP for 13 weeks. The increment in your vote value would be $0.4 as per current price of Steem. Let's say that you sell 5 full votes per day then people will send send 0.05-0.06 SBD to get your vote. Total of 0.27 SBD per day on average. You'll get about 0.23 SBD per day (85 % of the SBDs sent) and also the SP from curation. The price may rise and fall so you may get 0.1 to 0.4 SBD per day in these 13 weeks. Let's say average is 0.23 SBD.

Multiplying 0.23 with 91 results in 20.93 SBD just from vote selling. The votes you seldom do on your own posts and the curation rewards from other posts are added advantage. It will definitely be profitable, unless price of Steem plummets. In that case your vote worth will fall and so will the returns.