For what it's worth, I have one property in Rochester that I purchased in 2011 for $23k that returned 20% for a couple years in a row, and that $23k included a $8k finders fee (the actual sell price to my "finder" was $15k). Had I been knowledgeable/active enough to find the deal myself, it would have been more like a 30% return.
Major qualifiers:
1) The tenant never asked for anything significant to be repaired,
2) It's in what I'd consider a war zone
3) The tenant stopped paying rent and was evicted a few months ago, and now I'm probably paying for all the things that weren't repaired while they were living there, haha
Bottom line - 20-30% is possible, though there will likely be risks/extenuating circumstances. Most of my others are more like 12%-16% like Dave said, if they have a decent year without an eviction or significant vacancies. I'd say even to maintain those rates you need to be on your toes and serious about your business.