I think a cab would be expensive. my guess is $40. chances are, if you are taking a train to 30th, it also stops at suburban station, which is a better place to get off. if not, you can take one of the many trains going to suburban (almost all lines, but make sure it is GOING there and hasn't just come from there). the cost is $4, but it usually isn't enforced due to high traffic flow. if you are asked, you simply pay $4 (no crazy tickets or anything, as purchasing on-board is perfectly legal).
alternatively, from 30th street, catch the blue line subway to city hall (eastbound), and take a free transfer northbound and get off at the erie station stop.