The cost of A T Shirt Printing press can range from as low as $300.00 to as high as $3,000.00 depending on the size and the capabilities of the T Shirt Press.
Usually it's better to just have someone else print out the T Shirts and ship them to customers for you after you design them.
Check out
https://passiveshirtprofits.com as Lisa Irby has a great course on printing and selling T Shirts for profit.