DTG and dye sub are two totally different processes and two totally different inks. Although to the small dye sub shop ink can be very expensive if you are running wide format printers ink is very reasonable - what a small shop pays for 200ML a wide format shop will pay for a liter. Many printers such as the larger Epsons and the RIPS will actually tell you exactly how much ink you used to print the file thus allowing you to calculate you ink cost very accurately.