What kind of designs are you doing on your shirts? They may largely determine what your best low budget option is.
For single colour designs, I'd seriously look into PhotoEz (
http://www.ezscreenprint.com/). You can "cure" the inks using a home iron, or you can buy a fixer additive that will make the ink air cure.
For multi-colour designs on light coloured garments, heat press is probably your best bet. Unlimited design/colour potential for the same base cost.
Tell us a little more about exactly what you are doing, the kinds of designs and your target market.