ask five people their 'formula' on pricing and you'll get five different answers. obviously you need to know your costs, or at least a good ballpark figure. then, what is the competition charging? that's not to say you need to be charging the same or lower, just that charging twice as much as a comparable item probably won't get you very far in most cases. imo, it seems easier to charge for a brand than for a service with tons of variables that competition and customers add. those formulas mentioned mean nothing when charging for a brand shirt (sorry, guys) ~ charge what your market will bear given the kind of customer you have and their expectations of price, quality and service. if that fits into some kind of magic industry formula for a brand, i've yet to hear two people agree on it. if you were offering printing services, a lot depends upon the vagaries of the design, method, shirt (style, brand, size, and colour) and quantity, so again, the formula isn't exactly rock solid in every situation.
so, that's where i think you start, with your production costs and a good idea of what your direct competition is doing and their prices (this assumes the market you're aiming for is roughly the same).
i would say one thing not to do for your brand is go in under-funded. so many people have a brand that they think will sell 100 shirts a week just by having a free or super-cheap website linked to a facebook account. no one ever wants to pay for a professional web developer, facebook ads, seo or anything. but, you know what? those are some keys to selling stuff on the web right there, and as far as i can determine, it still takes money to make money. what, does everyone think the internet has gone soft on capitalism? lol. a poorly made youtube video is likely to do as much harm as good, eh?
something else not to do is assume you don't have to do any market research. some can get away with it more than others, but generally it's a durn good idea.
must my opinion, but don't give your customer any other impression other than you're doing just as good as you want to be doing. don't give them the idea that you're anything less than a consummate professional as evidenced in your web site, product, about us section, facebook/twitter/whatever, etc.. 15 typos on your landing page and i'm not even going to bother looking at the rest....
don't have sales on every single item you offer. all that tells me is you're slowly going out of business and are desperate. a discount for liking you on facebook if they enter some kind of code is fine, or a sale on retired designs or close-out stock is cool, just don't give the impression that you're low-balling yourself across the board. i then begin to think you're having a total liquidation sale.