Depends how much you want to make, lol. The thing is that you really need to make a sales call and look at the job...there goes 1/2 hour. Then you need to load up, drive there, unload, fire it up...there goes another half hour. The average small garden is only going to take an hour or so to double till. What's a couple hours of your time worth?
Oh yeah...then there's the people who put all their fall leaves in the garden...ever try to till a foot of wet leaves into a garden? lol Another problem is trying to work within a fence, as most have their gardens fenced. The handles of the tiller will hit the fence on corners, so you have to cut the corner and leave it untilled and either use a mini tiller or hand dig it. Then there's the tree roots that wrap up in your tines...gotta cut them out after each job and sometimes during them. Tree roots from large trees can be really long and end up in places you might not expect.
I don't mean to discourage you but after a few jobs I stopped offering it... and if it's a front tine tiller it's pretty rough on the body. The rear tine ones are nice but kind of long, and make sure whatever you do that you don't lean on the handles of a rear tine machine...they will get up and take off like a bat out of hell!!!!!! I had a front tine Toro but have used the rear tine ones as well.