What I almost did was buy a cheap printing machine and then I just bought shirts in bulk for like lower than $1 in total. Then just printed them myself. I didn't go through with it because I lost a need for the shirts, but based on what I remember, that would be the cheapest way to go, especially if you want to print shirts in the future.
Check out the catalog at http://www.digitalmayheminc.com
There are no prices in the catalog, but they are very wholesale-like prices... I would recommend the "Alternative" or "Bella" brand. They usually have a live chat on the website where you would be able to ask how much everything costs. Good Luck!