Hi Paul,
Now we know you are going to be doing mugs as a side business or an add on to an excisting busines then its worth a go.
I do embroidery and t-shirt printing and run mug and sublimation as an add on. You will be able to use the subli setup for your t.shirts too. You wont sell many mugs but it will pay for itself and be profitable.
What I would do is give each of your excisting customers a mug with their logo and yours on it. Almost guaranteed to get at least one order from this and that will pay for all the other mugs you have "given away". Xmas is coming and its the best time to do it.
Word of advice. Buy good quality equipment and inks. Dont muck around trying to find cheap , it does not work. Get a Ricoh printer and a good mug press, Sawgrass inks and a good paper.
My thoughts, I hope it helps. Good luck....