Once the initial expense has been paid out and providing you get a good trade there aren't too many extra outlays.
You've got your rent, rates, elec, maybe gas, insurance (if you bother) wages etc.........
So you need to keep pretty busy
Plus you've got to try and make some money for yourself and pay back any loans you needed in setting up initially.
I've actually looked into this and was (pipe dream)considering setting up a training place at the beginning of this year, but the building I had my eye on was taken and I'm too busy with work (crap job, crap pay etc).