No I wouldn't leave either, not that kind of money. I've been contemplating this for years and I guess I havent jumped ship because the numbers just rarely add up. I've been doing this part time for 6 years (with insurance/I pay the Feds and NYS). Everything has been tracked in quickbooks and I can say that many times that end number at the bottom of the report sheet isn't very good. Granted I've had plenty of fun spending money and have put it to good use but even with that the expenses add up. As someone else stated if I didn't have kids I'd be making a go at it. Knowing that there is soooo much more before and after the mower/truck/equipment is turned off may not be worth it. I think I'll just stick to the part time work and make a few extra bucks to help pay some bills, work down some debt, redo some of my house, and other things. Once the kiddos are our of daycare ($1000/month expense) and my truck is paid off we'll be doing well enough that I may just close up shop or make a few thousand in the summer months. But for now it helps close the gap on bills.
I do some lawn and landscape work for a friend of mine who is a vetrinarian. Many does he make some money. Now he works all sorts of "off" hours to accomadate working people, but he does quite well. He's also a 3rd generation vet. But no one in their right mind is going to take their pet to a "lowball" vet. All vets are expensive...
Most of my equipment is now paid for so the expenses are getting less and the profit will increase, but nothing lasts forever. If my current job paid me that salary you listed I'd close up shop TODAY. Mainly because I really like my job it just pays real bad compared to my industry and the past few years (having kids) we've needed some extra money.
Good luck with a decision, if you're still making one, and just think things through.