The going rate for a burger flipper in California is $8.00 an hour. In Wyoming, the minimum wage is $5.15. It's $2.13 if you're a waitress, but then, a waitress gets tips on top of her hourly wage. And really, in cases like that, the federal minimum wage of $7.25 applies. In the US of A, while states get to determine the minimum wage, they must meet at least federal standards set in Washington by Congress.
Here's where paying attention to politics and the news helps you.
The minimum wage is the same for old people and young people, experienced and inexperienced alike. In fact, the more experience you have, the more you get paid. The minimum wage means just that, that's the minimum you get paid, regardless of your level of experience. Nobody walks into McDonald's and works for free hoping to get enough publicity to become the next Gordon Ramsay.
Yet for artists, it's a different story. If you're an artist, you'll often hear something similar to "there's no pay, but it's good experience and for your resume".
The 16-year old with no experience who applies to McDonald's gets experience, something for his resume, and the minimum wage.
There is no reason for us, as artists, to give our work away for free in hopes of gaining experience, publicity, or any other promises. Those of us who have experience can charge more than the minimum wage. But those without should be charging at least the minimum wage. If someone is making money off our drawings, it only makes sense we should too. After all, we drew it. If we don't value what we do, no one else will.
Further, your decision to work for free actually hurts commercial art as a profession making it more difficult not only for other artists but yourself to make a living doing what you love: drawing. Because when you start charging for what you do, the people seeking drawings will simply go to other artists who are willing to work for free. When you and every other artist starts charging for drawing, it becomes possible to make your passion a "real" job.
And if for some reason you feel compelled to work for free, draw for yourself. Work on your own comics, tattoos, and cartoons. At least if it takes off, you'll earn money from the sales directly, and you'll have final say in what the people see.
So now that you are charging for your work, be it $80/hour or $8/hour, how much do you charge for that tattoo your friend wants?
You figure out how long it will take you, in hours (15 minutes = .25 hours, 30 minutes = .5 hours, 45 minutes = .75 hours, etc) and multiply it by the rate you charge (minimum wage at least). That gives you a number. That number is what you charge.
So for that tattoo, including revisions (because everyone wants something that will be permanent to be as close to perfect as possible, whether or not they pay for it), lets make an educated guess that it will take you about 20 hours. That is, an hour or two to come up with several thumbnails (or super tiny, rough sketches of ideas), about 4-5 hours to come up with a big clean drawing of that thumbnail, another 2 hours or so to ink it, and another 2-3 hours to color it. That right there is 12 hours. Of course, your friend or client will want changes done. It sounds cynical, but it's a fact. Sometimes the changes will be small, sometimes they will be big. But changes, big or small, take time to make. Your time. And you deserve to be paid for your time.
You did not get anything wrong. It is impossible for you to read their mind. Communicating their vision is what you are paid for. Do not feel guilty for charging for revisions. Sometimes clients will see what you've done and get new ideas that they'll want you to incorporate. It's part of the process. It's natural. Sometimes frustrating. And always worthy of pay.
That said, you may finish the job in 15 hours. You may finish in 22 hours. Either way, 20 hours is a good guess that comes from experience. Do not be afraid to make mistakes in guessing how long a project will take. We all learn from our mistakes. But a good tip is overestimate to be safe. Any time you save, you can either choose to give the money back or keep as a bonus for working fast and efficiently. I tell all my clients that if it takes me less time than I say it will, I'll charge less. And if it takes me more time, I'll charge more at the same hourly rate. My quote is simply an estimate, like that of a mechanic.
Lets say that tattoo job for your friend took you 15 hours and 30 minutes. At $7.25 an hour (the federal minimum wage), that's $112.38 (15.5 X 7.25). See, paying attention in math class helps.
Of course, since it's a friend, you might do it for free, or as a birthday/Christmas present, or in exchange for a date with his cute sister, or trade for some beers or a few personal training sessions (imagine your friend is a gym rat). You might even charge $112.38 because if it was for a stranger, you might charge $465 (15.5hrs X $30). But the tattoo you do still has some worth, not only to your friend, but to you as well. And that worth is the time it takes you multiplied by your hourly rate.
For a recent gig, I was asked to storyboard and animate an intro for a corporate website.
I won't say how much I charged or was paid, but I will say that though I worked from home, I kept regular working hours: 9-5, Monday through Friday, saving weekends and evenings for friends, family, and my girlfriend. Yes, sometimes I put in overtime to finish by my deadline. And when I went past my 3-week deadline, I charged for the hours I went over. Because I charged for it, I was able to focus on doing it full time. I was able to put forth my best effort without having to worry about other sources of income to pay necessities like rent, food, clothing, etc. I would not have been able to do my best work at the time if I had not been able to focus. I would not have been able to focus if I was not paid for it. The hours and weekends I did not work on it allowed me to enjoy life and come back refreshed and with a renewed sense of dedication and passion. It kept me from hating my job and my employer.
Do your profession a favor. Charge for your work. Even if you have no experience, the least you deserve to get paid is the federal minimum wage. Your clients are willing to pay their mechanic, their doctor, and their barista. They need to pay you as well. And for the sake of having a future as a professional artist, you need to charge.
Never work for free.