Your local printer may base his/her prices on a Ouija board or Magic 8 Ball,
but more likely it is based on the cost of the garment you selected and the
roughly estimated cost for the printer to print it (and a little profit).
The shirts ain't cheap and anything more than a white large is gonna cost ya!
The printer buys in volume that would clothe the population of China. VOLUME VOLUME VOLUME! They can get 'em cheaper than you.
Plus, the nature of the beast makes storage an issue. Shirts are bulky as shit.
Printing shirts requires films and screen. This costs money, which may be paid for as an itemized item or factored into the overall ball of wax.
You will get charged per shirt. More colors, more difficulty and fewer shirts to amortize the above.........you got it baby, more dough.
Dark shirts are more because they require an extra step, drying or an extra color under the others (usually white).
Some common pricing misconceptions:
Five jobs of one hundred are not the same as one job of five hundred. Discounts come almost entirely from efficiency of printing, not from volume purchasing.
Printing the backs of the shirts is no small matter. The back printing is virtually like another entire job and usually will be priced accordingly.
You can't just throw a couple black shirts in with a white shirt order and expect a similar price for them. Printing on dark shirts usually means using different screen meshes in addition to the shirts costing more and so also they need to be priced as though it is a separate job.
Stick with these basic premises and you should be ready to go buy some shirts, baby.