Upvoting costs you nothing. It's how you rewards others for their content. The value of your upvote is based on how much steem power you have. Voting power is a lot like a fuel gage for your upvote value. So if your VP is at 100% that means your upvote pays the max amount that your upvote is worth. Your VP goes down the more you upvote. When you are not upvoting, your VP will go back to 100% over time. Make sense?