After years of doing commissions, I've found that there are three main obstacles that prevent you from making any money as an artist:
1.) Having too high ambitions. You're not going to be making millions of dollars right off the bat, no matter if you're doing comics or children's book illustrations, etc. Having unrealistic expectations like that can set you up for failure, and often times artists just think they're "no good" and just don't realize that their ambitions were set a bit too high, then give up. Be realistic with your goals; come up with a 5 year or 10 year plan that you can follow, or even just a grocery type list of goals, and don't expect fame and fortune the moment you finish art school. Which brings me to the second point:
2.) People waiting until they're done after art school - which could be in a year, two years, or up to 4 - 6 years - before they try and make money with their art. Do not wait. I started doing commissions as early as the tenth grade, and by grade 12 I was doing work for magazines and even had a finished children's book under my belt, which I got paid $300 to do. Don't think you're not "good enough" to do commissions yet. In your own view, you're never going to be good enough. So you may as well just suck it up and try it out, and at least improve in the process, than constantly wait and wait and wait and never be satisfied enough to start making money. Start making connections as soon as you can; like the music industry, you need these connections to meet more people and get more projects. Don't wait until you're finished of art school and thousands of dollars in debt to do this.
3.) People selling themselves short. They either give work away for free, or constantly give out discounts, or just don't price themselves they way they're worth. Doing free drawings for family members is one thing, but doing drawings for $5 an hour because you're too worried to ask for more is just going to do two things - you're never going to be able to make a living, and you'll be doing more work for less value; and you'll just keep giving clients that reason to say "well, you'll get recognition!". That doesn't pay the bills. You wouldn't work at Starbucks or McDonald's for any less than $10 an hour, or whatever minimum wage is in your country/state/city; don't undermine the value of your work, which is unique, and a lot more special than flipping burgers and making coffee. I charge between $25 - $30 an hour depending on the project. Is it a lot? For some people, yes, but I've run into people who have paid me $200 just to write a chapter for their books. There are people who are willing to spend money, so don't sell yourself short, and be clear to people how much your work is worth. Be flexible if you want, but not submissive, and don't go so low that you're losing money, especially if you use a lot of materials in your work (ex. paint, canvases, etc.)
So that's my contributing post on the matter u o u ~