Coming from someone who operates a medium-sized sign shop, we also have a full screen printing custom apparel department that keeps 2 full-time employees busy every day, it is a CUT-THROAT business to say the least. As mentioned above, there is always some hack in a garage/basement picking off some of your orders by low-balling everything. It is annoying to say the least, but screen printing is NOT easy, if you care about a quality print, not to mention the fact that quality ARTWORK is just as important. Any monkey with a computer and coreldraw can output an image to a screen, sell the quality of your work and don't treat it like a commodity. Depending on your market, business model and overhead, it will probably still take a couple of years to get up to speed. There is money to be made in the business, but like everything else, you get what you put in and it will take a while to learn the ins and outs of the printing.
IMO it is incredibly difficult to set your prices before you really know what is involved, waste factors, setup/cleanup time etc etc. To start out, maybe check prices on a web-based business like customink.com and set that as your starting point. It is not wise to base your prices off of someone else's overhead, but this will at least get you rolling. We usually include 15-20 minutes of artwork/setup in every job, but we never give away a custom shirt design (anything that takes us longer than 20-30 min to design), it is always factored in somewhere (we charge $75/hr, though I guess the screen-printing industry standard is $30/hr, which I never understood).
We obsess over the look and feel of the printed garment. Using plastisol inks and getting a soft "hand" is not easy. Figure this part out and you will already by lightyears ahead of the basement/garage hobby guys. OR, just use vinyl heat transfers but you will quickly find out how limited the process is. I don't own a DTG printer but I have considered them on multiple occasions.... expensive machines that are very expensive to run and maintain in contrast to a screen printing press. We started with a very small 3 color 1 station press and a flash dryer. Look around on craigslist for the guys selling off their equipment after they realized it wasn't easy and you could probably save yourself hundreds of dollars on a modest starter kit. Good luck!