If you were to sit and work it out it would be pennies per coaster, tbh I wouldn't get hung up on it. My printer has been working for a year now on standard carts and is only now showing low ink on 3 out of 4....still life left in these anyway. I've done the equivalent of a few thousand coasters! Certainly been through 500+pages of subli paper :-)
Press wise. T-Shirts you really need a larger A3 size press, everything else you'd get away with an A4 size or similar. Obviously you'd need an A3 printer the n though so costs will rise significantly....unless you're going to print your Tees with vinyl?