Les,
For a conservative number, take 90 trips per month, times TR4A's $-per-trip figures.
If you don't live close to a base, if you don't like to pick up lots extra, but instead just fly your line, 90 is probably a pretty good number. You'll get perdiem on top of that, which will up the total some, although for at least the first year, you'll have some payroll deductions for uniforms & such that take a bite out of your paycheck.
If you need really precise "knat's behind" sort of numbers, you need to talk to somebody at SWAPA, because "the average" is really just the mean of the 138/month guys, a bunch of "pick up a bit most months" guys, a bunch of "fly my line & go home" guys, and a few "give away more than I pick up" people. What all that averages out to doesn't really have much bearing on your situation; if you don't plan on picking up a lot, use 90 trips and go from there. If you think you will pick up a bunch your first year, add another 15 trips at 2nd year rates & you're probably in the ballpark.
Happy Thanksgiving!
Snoopy