I got a Betty Crocker borosilicate glass casserole baking pan at the Dollar store. I've used it several times, and it hasn't broken. If a Dollar store can sell glass cookware at a profit, how many fractional pennies did they save by making these glass mugs?
To be fair, some types of glass are very good at handling rapid temperature changes, at least in the range you'd find cooking. Why they wouldn't use that glass is beyond me.