We come up with a tool expense based on the past years spending for tools. Not only do they wear out but they get left on the job, stolen etc. I think to rely on your history to project the future is a fair assumption and safe bet. Adding guys will add to your projection accordingly.
RE tools life. I've got a porter cable "Tiger Saw" reciprocating saw which I bought in 1996, the year I got my license. I've still got it. My first DeWalt cordless got left on a job after 3 or 4 years use and was gone when I went to retreive it.
Hope this helps...
