@Clarence Harris I was in your shoes a year ago. Now that I have two flips and a few properties that have required work under my belt, I've become a lot more comfortable with the rehab prices.
What you're trying to do is completely skip the learning stage of this business. While we all want to do it, unfortunately, there is no easy way. You really only learn when you have something to lose.
What I'd recommend is walking your potential flips with a contractor and asking questions. Most will have a "per sq ft" type number for labor and then you can do your own research on materials cost. Most materials cost is minimal as the real cost comes from labor (just like going to an auto shop).
At some point, you're just going to have to take on a rehab and learn while doing. If this isn't comforting to you, then find someone locally who's done a few rehabs that will give you an honest opinion on a general number of what the rehab should cost. Make sure you're all-in is about 70-75% of the ARV and then buy!