What is the community's general attitude toward tattoos? If you live in a bible belt area or somewhere that generally opposes body art, there is a good chance that you're not going to get a lot of business. If you're in a more open-minded community, you may be busier.
How busy of a town/city is it? Population has a lot to do with how well a business does in general. If you're in a small country town with more dirt roads than high-rises, business is probably going to be a lot slower than a shop in the middle of a big city. How much competition will you have? If you're in a city that already has 10 tattoo shops to every 100 customers, you may do less business unless you happen to be working in one of the most popular shops available.
How good are you? If you're highly skilled and in constant demand for your work, you're going to make a lot more than an artist that only knows how to color inside the lines. So, as you can see, it's very difficult to answer how much a tattoo artist can make. For many of them, I'd say $20,000/year is very possible. Then again, artists who are in high demand could make well into six figures. Sometimes it's a gamble, getting into a business when you don't really know the outcome and whether or not it will pay off. Passion means taking risks, and art is all about passion.