Great questions, Lawrence.
In my experience using Sponsored Updates, they did an effective job at driving traffic to our website (this was at my last job). The targeting was very good and I noticed a traffic boost attributable to these.
I think PPC is a good starting place to find a baseline. It lets you pretty clearly see how interested people are in your offer. CPM is more about impressions/brand building IMO.
As for what should you see each day, there is no external or objective answer to use. You have test it for a period that your budget allows and see how it performs for you. To make your test valid, plan a month's worth of sponsored updates against your regular status updates and test your offer with different language (A/B) to see if you can determine any phrasing that brings a measurable lift or not. Document it clearly with the hypothesis(es) that you want to test as well so you know if you are getting what you want. That is, "If I use 'free' versus 'complimentary' will that increase yield?" Some audiences respond differently to those two words.
This is one of those efforts that you won't know what you will get until you try it and focus on it. After you go through several rounds, you should have enough insight that you know what works and what doesn't, then you can segue into auto-pilot mode with those offers that are proven to work. Next you'll want to test new offers the same way, building a library of reusable ones. This can also inform any other type of paid promotion you do such as Google AdWords, etc.
Hope that helps.