Unlike before were returns was solely base on curation reward and also author rewards that if you self voted, but now we have different means in place in which an investor can get good return which includes
2 Curation reward
3 Author reward/self voting and
4 Vote selling
Delegation and vote selling are further divided into various mean to get returns
Say delegation, it could be to projects running on the steem blockchain or to a particular user who pays certain amount within or after a certain period of time.
Vote selling, this as to do with exchange of steem for your upvote.
So in general it would be impossible to actually calculate the amount you stand to gain when you invest in steempower but what is certain is that you stand to gain a lot and your investment still intact