How do you calculate CapEx on your rentals?

[Fritz Liedtke]

What is the prevailing wisdom (if there is such a thing) on calculating for future capital expenditures (CapEx) and maintenance for SFH rental properties, when running the numbers on a potential property?

Some people say $150/mo (I think @Brandon Turner does in his book), while others suggest a % of the purchase price.  I don't know how a % of the purchase price would be correct, since a 1000SF house that costs $50K is going to need just as much maintenance/capex as one that costs $150K.

For a SFH that is in reasonably good condition at purchase, what is a realistic amount to put in your calculations for reserves?