How do you research the buy vs. build cost in a given area?

[John Belson]

What is the best way to understand the cost of building a home vs. buying one? I'm new to the game, but I've already seen tons of times where this knowledge would have helped. Some examples:

1. Plot with great location, questionable home. Is it worth it to demolish and build a new one?

2. Comparing two possible investments, one with an existing home, one that's a vacant plot.

3. Buying myself a new home. :)

Obviously, the only way to get concrete answers will be to sit down with builders and go over specific plans. However, I don't want to start annoying my future business partners by running around asking hypothetical questions, taking up hours of their precious time when I may not even end up building.

How can I get a basic, rule of thumb understanding so that I can make the big decisions before I move on to the details?