No, it costs you Voting Power, not Steem Power. Each account get 100 % voting Power, and each vote with 100 % costs 0,5 % of this Voting Power. So you can cast 40 votes and get down to 80 % Voting Power, or 80 votes to get down to 60 %. At lower Voting Power, each vote is worth less. However, you get +20 % Voting Power each day, so you get to cast 40 votes daily without getting to low Voting Power.
Steem Power is a different thing, and mainly affects how much your votes are worth.
Also note that the percentages for voting power will change in the next hardfork, and after this you only need to cast 10 votes to drain the daily 20 % of the Voting Power you have, so if you want to keep voting 40 votes daily you need to lower the vote to 25 %.
Hope this makes sense to you, but let me know if you got any questions, and I will try to answer them as best as I can ;)