It really depends on how much Power you have and how whole SteemIt community will grow. Basically, more you invest in your SP, more influential (and higher rewards) you get. I'm trying to find and upvote good articles by myself, and ROI seems to be now (~ month I'm on SteemIt) around 10% from curation rewards and post votes on my blogs (all of it are just cents). System is set so more active over longer period you are, more rewards you will get - e.g. you can get curation rewards from your upvotes more than once (as more people are upvoting over longer period you are getting payments over that period). Lets discuss after 6 months lets say. I believe it should be at least around 20%. That ROI I'm talking in Steem (just to clarify), so if cryptos themselves will be 2x higher, ROI in USDs might be 2x that much.