Determining asking prices on small multifamily property (2-4 units)

I've been researching properties and trying to determine good asking prices. As part of my research/practice I've been using the method described in The ABCs of Real Estate Investing (great book BTW) to determine prices using the formula Purchase Price=NOI/Cap Rate. This method has given me crazy low asking prices on the properties I've practiced with. My question is, does this formula still hold for small multi family (2-4 units) properties? In the book he uses it to determine asking prices for larger multifamily properties.

I've only used it on properties I've been finding listed on Realtor.com/Craigslist so maybe these numbers are so far off because I'm looking at properties that are already listed well above a good price. I'm trying to be very conservative with my expense estimations. I guessed 10% for Cap ex, 10% for property management, 10% for general maintanence, $650 per unit for insurance, and the county tax assessor website to get actual taxes. Am I guessing too high the expenses leading to an NOI that is too low? Also, is there a good way to estimate cap rates for a neighborhood other than speaking to a realtor?