How can I learn renovation costs?

[Justin Megna]

I'm extremely interested in learning the BRRRR method. I already have experience researching ARV through comps and rental rates. But I feel completely ignorant when it comes to estimating renovation costs. How can I learn to make reliable renovation estimates with only a house walkthrough? I expect I can't have contractors give me estimates for a house I don't already own, and I can't know if a house is a good buy unless I have a reliable renovation estimate. So how can I learn to estimate renovation costs?