When researching keywords on the various sites like Google adwords or wordtracker et.al., What's the minimum number of daily searches that you would consider creating a niche site about?
The thing is some of the tools, well all of them actually are showing inflated results because the numbers become inflated with webmasters doing research. So even a minimum that someone may suggest, may be a complete waste of time and explain why some sites get no traffic for a keyword or phrase after targetting it.
I agree with Pipes and Hesheys, You should try to use various combination of keywords instead of targeting just a one or two. Minimum of 5 combination would be good and target 5 or around keywords in a page.
It partly depends on the margin of however you're monetizing the site. A site where a single sale gives you $1,000 profit needs far less traffic to make it worth while than a site that gives you $10 per sale. But in general, I'd shoot for average search volume in the Google keyword tool for the site concept keywords. Individual pages would be related to those, but can be drawn from lower traffic keywords if necessary.
If you are selling products then just target your audience. Fewer clicks that are targeted will win the day if your selling to a select few. If however your trying to run an ad-words campaign and generate revenue from the clicks off your site - you had better aim high and be sure that your tapping into a good stream of traffic.