I'm a professional longarm quilter. I did a lot of research before buying my machine.
It cost about $20,000 to get started, between the longarm, battings, thread (I started with 2 cones each of 100 colors), lighting for the workroom, books, pantographs, business cards, website, insurance, and my state business registration.
I wasn't taking clients for the fist few months because I was busily quilting every day for practice on Project Linus tops. It took me 2 years to earn that initial investment money back.
I was lucky enough to know many quiltmakers and get a good business going. I am very busy with client work, I have a huge client base now, and I still deeply love the process.
But the work is seasonal (less business in January and August, 7 days a week from October till Christmas), and it is hard on the body(standing and stretching and lifting).
At the end of the day, after paying expenses, taxes and health care and insurance, it doesn't pay as much as I had expected. I would suggest doing a lot of research before deciding, seeing an accountant, and seriously take every expense into consideration.