Why? Economically it seldom pays, and then only if you have invested in good equipment. $400 won't get you there. Low profit margin, high accident potential, heavy manual labor, and depending on your area could be highly competitive. I think the rest of us cut wood simply because we like to and it's a relatively healthy occupation.
You can't, and never should, do it alone either, it's just too dangerous. You need other cheap reliable labor, which is hard to find, hard to keep, and harder to manage. Further, the cost of saws is negligible compared to the operational costs of transportation and fuel. And never, never forget the safety equipment. It's incredibly low cost compared to the cost of even the smallest accident, and there will be some even if you work with trained professionals. A good splitter will be required even if you just cut firewood for yourself, and that alone is far more expensive than a saw collection if you need volume.
Not trying to talk you out of it, but there are many considerations that go into any business plan. There is a very good reason that many tree service businesses do not process firewood themselves, especially since it's so low-margin and labor intensive.
On the other hand, it's an easy start-up business, especially if you have most of the equipment already and a free near unlimited supply of good hardwood trees.
If you are just starting out, even if it's for personal firewood use, I suggest you get at least two saws, a big one for felling and bucking, and a smaller one for limbing and climbing. That also gives you a back-up saw if one of them breaks down, and they will.
For big saws, any of the larger Stihls, Huskys, Echos, or even the newer Efcos are good considerations. It should be at least over 60ccs with at least a 20in. bar or more, which tend to be pricey. Avoid the Husky Rancher, the biggest saw sold at Lowes. It's just not that good - I understand from my repair shop that the rings tend to stick in the exhaust port and destroy the saw. Anything in the Husky XP line is a good choice but can easily cost over $1k. I also have a lot of respect for the bigger Stihls as well as I have used them
Don't discount the great values that can be found in used saws. I picked up a brand new "used" Husky XP372 last year with a $300 discount at a local dealer. It was never out of the box, sat on a shelf for three years at a company that bought it at retail, and was traded in for a newer saw with the dealer. Already out of warranty, but warranties are practically worthless anyway as many on this site already know.
For a smaller saw I went really cheap. Bought a Poulan Pro 4618 from Lowes at $160, on the theory that I could buy two of those for the $320 cost of the 18" Husky, even if it was crap, as are the smaller Huskys. Since Husky owns Poulan and they are manufactured at the Husky plant, figured some parts and engineering have to be the same. Somebody please correct me if you know different.
Did not stop there. Immediately modified the exhaust (instructions are on this site) and re-tuned the saw (voids the warranty, but who cares), then refitted it with a 14in. professional bar of harder steel and a full chisel chain. At that point it was still under $210 and a real banshee of a saw for three years now, without a single breakdown after about 20+ cut cords (about 60 trees).
Recently worked alongside a large professional tree cutting team doing a wide clear cut for a major power project, and they were in absolute awe of what that little saw could do. Never seen anything like it. Small, lightweight, easy to handle, doesn't tire you out, cuts incredibly fast, and never slows down, but it does scream. It's a 42CC saw that doesn't know its small.
Good luck, whatever you do.
Husky 372XP
Poulan Pro 4618
Poulan Wild Thing (it's a joke, but it still runs, just don't use it. May be a collectors item as the worst saw on the planet. Your electric kitchen knife is a better saw)