Gn, kook, GM and slap own most of the business. The rest tbh are all small brands (some bigger/more established). Most are strong locally in their area then fall away, only strong if they get someone as an agent who helps sell them etc.
From my limited growing experience, it's not that hard to start a brand of you want to. You just have to accept its either going to take ages and a fair bit of start up capital or lots of start up capital and less time. Wages wise, again.. Depends what you want out of it.. If you truely love thur job (be it bat making or retail in the industry) you can make a living quite easily, however if you want flash cars, big houses, flash trips etc... Then it's probably reserved for just the big boys.
All in all, from what I've made in just a few months it suggests it's thee but you have to have the capital to do it (this is really the limiting factor, not skill or knowledge really)