to add to what joker said..its depends on the company who's buying the work and the volume and expected retail prices.
Also as a commercial artist when you are approaching a client..or a client approaches you to do artwork for them usually you submit several designs. Sometimes the price for just straight out buying existing artwork is cheaper then asking you to develop new work. just think ..your average shirt that retails for 30-35 bucks cost anywhere between 6-14 dollars to produce depending on the size of the run, amount of screens, quality of shirt, cut and sew shirt..etc.
when it comes to amount youre expecting to get you have to establish a reasonable idea of what your artwork and time is worth, but also...consider that getting someone to produce, sell and have people eventually wear your artwork is also always positive. If youre just starting out its good to take advantage of every opportunity..in all reality..all of these trends with t-shirt "art" and "cool" graphics is very fleeting so overall getting things produced and learning from your experiences is an overall positive move.