It depends on a lot of things plus over time it will change. The "push" of the steem power in 24 hours due to the total amount being released every 24 hours and then the decline (remember Steem is 90% pre-mined). So over time you will need more and then on a less popular day (like a week day when less people are on) you'll get paid more for the same amount of steempower

https://steem.io/SteemWhitePaper.pdf