How do you estimate costs to analyze past sales?

[Philip Tretola]

I am just starting to look at potnetial opportunites and I have been looking through past sales in MLS from several neighborhoods that I am thinking of investing. Without a solid grasp on what expenses and other costs for each property, how do you come up with ballpark measure of past sales to understand neighborhood opportunities better? Should one just estimate costs and work off those estimates? Anyone who has any thoughts on this subject, your feedback/guidance would be greatly appreciated.