The monthly cost to operate just the machine is nearly nothing. They don't draw much more than the rated wattage and that only when running full throttle.
Cost to run the business is a different thing. I do mostly wholesale and my customers supply most everything so it's mostly artwork fees and laser time for me. I go through a can of cermark every three months or so. I charge an average of $60 and hour but don't really have a minimum charge since I'm doing 2 or 3 thousand parts at a time.
I tried to write a business plan using Business Plan Pro but never finished it. At this point, I sometimes wish I had never gotten started, it's cost me $60,000 to start what is basically a hobby and that's money I'll never get back. Marketing my service was very hard for me, I'm not all that out going and I could never come up with viable ways to drum up interest. I finally gave up and have 4 customers that might bring me $2500 a year if I'm real lucky.
70W Epilog Legend 24, bought it new in December 2002