My 2 cents.
Selling a shirt/clothing line works at two sizes.
#1 - Small/hobby guy, print a few sell them direct, make some money.
#2 - Huge company with great products and "mountain" of inventory. You don't want to be setting up a 6 colour back and 3 colour front just because someone ordered 8 shirts, so you really need them in stock. You also need to do large enough runs (Think 250pcs. minimum of a design, and more realistically 1000+pcs) so complex multi-colour prints are still cheap to keep your margins high.
Getting from #1 to #2 is the trick. Having success at #1 is relatively easy if you've got good ideas/art. Building a few hundred thousand dollars of inventory and developing distribution while retaining a decent profit to get to #2 is beyond the abilities of 99.9% of people out there. If you're a creative type, #2 probably isn't for you. People who get to #2 are business types with money and connections, they employ creative types.
Good print shops will make people "starting a t-shirt line" pay cash up front because the vast majority don't work out, and someone gets left holding the bag.
I don't know too many people who:
1 - Can run a large, effective production screenprint shop and,
2 - Have the creative and artistic ability to produce artwork that will be in demand and,
3 - Have the business savvy and connections to build a large and effective network of dealers/distributors, and
4 - Have deep pockets to build stock so it's all ready to ship at a moments notice.
#1 guy can make $10 -$15 a shirt if they have their head screwed on straight, but will be limited by how much they can sell because they are doing it all themselves.
#2 guy will probably realize $2 -$4 a shirt after paying all the employees, advertising, marketing and overhead. Remember most people along the line will want to close-to-double their money, so a $25 shirt on the shelf will sell to the shop for $10-$15, and sell to the distributor for $5-$7.50. Take out the $1.80 - $3.00 for the cost of a decent shirt if you have case-lot pricing from a wholesaler, then take out the cost of the printing (say $1.00 - $3.25), and what you have left is your profit, and every link in the chain has to be solid, anything breaks down and that inventory can sink you in a hurry. Inventory costs you money just sitting there, it costs a lot more than most people realize.