You can send any amount of SBD above the minimum and once the bot account reaches 100% voting power it splits a 100% upvote between all of the bids based on each bid's % of the total amount bid during that period.
It takes 2.4 hours for voting power to return to 100% after a 100% upvote so each bidding period typically lasts roughly 2.4 hours.
Take the following example: With a 2.4 hour bidding period user A sends $1 to a voting bot and user B sends $3. The total bid is $4 so that means that user A will get a 25% upvote on their post and user B will get a 75% upvote. If the bot's 100% upvote is worth more than $4 then both user A and B will receive an upvote worth more than they have spent.
Please note that vote values are based on the current value of STEEM. As STEEM prices rise or fall the value of votes will too. Additionally 25% of the value of votes cast more than 30 minutes after a post has been made go to the curation rewards, so for a $0.75 payment you would need at least a $1.00 upvote to be profitable if you are taking curation rewards into account.
The developer of Bid-Based Voting Bot: 
The #1 place for information on voting bots and other options to profitably promote your posts on Steem: Steem Upvote Bot Tracker
