Figuring out what to charge is definitely a hard question to wrap around, that I’ve been asking a lot lately too!
We can often “undersell” ourselves because we just want to do the work, and we want to help people, and we’re afraid that if we put too high of a number out there that people will balk and we won’t get the work in the first place. Trust me, I’ve learned from experience, that if you put a fair number out there and someone balks, you don’t want their business anyway.
If someone isn’t willing to pay a fair rate for your work, that means they don’t understand the value of what you’re offering. In my experience that means they’re likely to keep asking for more as you go, and that they’re going to be more likely to ask for ridiculous things because they don’t know any better. Then, if they’ve got a knack for sales/negotiation, when you say they can’t have something ridiculous, they may try to use that as leverage to get a lower rate for what’s reasonable… since you can’t do that other thing you must not be that great. It’s annoying and tricky BS but people do it.
Therefore, figure out what your time is worth to you to offer to people, and then figure an overhead margin to cover your taxes, expenses, and surprises, and charge the resulting rate. In the professional services industries I’m familiar with, this rate varies from about 2.5 to 3.5x what the actual “take-home pay” of a given employee is at a small/medium firm, so for example I know a civil engineering firm here locally and in talking with the company president, who’s a family friend of mine, their ratio is about ~3x, so if they’re paying their guy ~$30 an hour, they’re billing out ~$90 an hour. That covers their overhead of HR, advertising, bidding jobs, etc… I’m friends with one of the Principals at a product development firm in Richmond and for them it’s 3x as well (I just called him this morning to check while writing this post because I was curious ;-))
I know several freelancers whom I’ve worked with who charge about 2.5x, and it goes up in larger firms, especially in the public sector. I met someone a while ago, probably 2 years ago, who does logistics stuff with Booz Allen and her time was charged out at $400 per hour, and I would bet she was making ~50-60k a year.
I’ve found that it’s also important not just to know what to charge, but how to interact with clients; how to respect what you do and bring to the table, so that they will too. I don’t know if you’re coming from another design field and already know this ‘part of the job’, or if you’re already familiar with Mike Monteiro—but I have found his talks and writings on this topic to be both a) irreverent and quite funny and b) on the money and very helpful.
Honestly, what’s more valuable than your machining time, is your design and process time. The machining time is a near-commodity utility, but knowing how to get great results out of it is not. Hope that helps!