What are the most expensive repairs?

I've really been thinking over what mikeOH has been saying about "Do the work yourself, and the management yourself"

At first I can't say I agreed with him completely, but as time goes on I'm learning more and more what he says is true.

Anyway, I started thinking about where I spent my money this year, and most of my money was spent on furnace installation. So, I'm thinking now, MAYBE I should start to learn to do some of the bigger stuff myself.

I can get all the required education, ect for HVAC for about 1/3rd the cost I spent this year on having someone else do it. Maybe furnace installation isn't the best thing, But i realized that I overpaid on just the materials by about double.

What would you say are the most expensive costs with rentals, or property rehab? I know one guy on here said he normally does all his roofing.

Thanks!