I'm in the same boat as Overloaded. I had a lathe/milling machine and converted it to CNC as part of my retirement hobby - making miniature railway equipment. I could do with a lathe that I could program to do the reptetive cutting (leaving me free to do something else) and attend to it only for the final acurate cuts. As it is, with Mach, I got both, any problems are with the lathe, not Mach.
The building and development of the CNC lathe has been a pleasure and quite absorbing, and in the scheme of things, not too expensive. I am prepared to "write off" the cost of it (and the time) as an education to me.
We now come to "making it pay". It depends on what your aspirations are. I would love to buy some of the CAD CAM programs everybody keeps talking about, and electronics being another of my hobbies, I would love a circuit board cutiing program - but - BLESS ME - people want money for them. Some has to pay.
If you are turning to the workshop for someone else, then you must charge by the hour for the time you put in for them and I am comfortable with £20 per hour - this is below the going rate in UK for someones time - but I am happy with it. Providing you are satisfied that you have worked efficiently (although not everybody bothers about that) then this provides a base for charging. Yes - don't forget programing time - my father-in-law could not understand why I had to spend a couple of hours in my office, prior to turning on the lathe, because in his day they reached for the hand saw and got on with it.
On top of that, you must ask for some costs for wear and tear - depreciation in business terms - because your machine will need replacement at some time - and repairing before that (I've already blown one set of drivers). Generally in business this is costed at 20% of the value of assetts (per annum) for tax purposes - so how much do you want to charge your customers for this - it should be something - say a weeks depreciation? because if you're are having to work, it has spolied the week.

On top of that there is improvement - there is always someway to improve what you are doing - better program, (not Mach), better anything - a bit difficult to quantify unless you have something in mind ( or a contigency fund).
And last, but by no means least - add 50% for being able to do it. You taught yourself, you built the machine, and the customer would not have come if he could have done it himself. A panel beater who makes my railway bodies (and charges well for them) suddenly said, one day, "Jim, you've got a CNC lathe, haven't you" - Ah, I thought ,pay back time.
At the end of the day - he can only say No - you haven't lost anything, and you can justify what you are asking (to yourself, if nobody else).
Above all - Be Happy With It
