Despite the criticism, I went ahead and ordered a complete brush kit from Sigma. Shipping to British Columbia, Canada is $14, so I'll be paying around $114.40 CAD before taxes and duties (crossing the border), which is still significantly less than paying $404.50+tax for all of the equivalent MAC brushes. For me personally, it's worth the risk, even if I have problems with some of the brushes.
As far as price increases, a company doesn't have to justify them at all, they can charge whatever they want for their product. It's really up to the consumer to purchase or not purchase from them. Obviously, if a company's product demand goes up, they will increase the price to make more profit, this is simply business practice. The purpose of a business is to make PROFIT. Someone has already mentioned that MAC increases their prices every year, does it really cost them more to produce their products? Not necessarily, but it's built into their policy to increase prices periodically. Is that illegal? No.
I am in no way defending Sigma, and if you think they have shady business or marketing practices, then you don't have to purchase from them, that is your choice as a consumer. But remember, business and marketing are shady to begin with, they are trying to convince you to buy their product at the highest price possible so they can make a profit. Some will try to make a profit at any cost *Cough, sub-prime mortgages, cough*