cabs do wait at the station, and it's a very easy station to navigate for its size. You want to exit on the side labeled "30th Street" to find them: if you're coming in on SEPTA, descend the ramp and turn right out any set of doors you see... if you're coming in on Amtrak/NJT, just locate the doors that are opposite the big angel statue. As soon as you're outside of the building you'll see the cabs.
As for cost, I can't really say... I've always driven to the zoo, but my guess would be around $10 each way. Just so you know, the cabs don't line up outside the zoo, so you might have a bit of a wait on the way back.
If you're feeling adventurous, to get back to 30th Street Station, you can hop on the 15 bus (buy tokens at 30th Street for $1.60 each, ask for a $.60 transfer on the bus), then transfer on the Broad Street Subway (southbound) at Broad Street, then get off at City Hall station and transfer (for free) to the Market Frankford Line (westbound)... then get off at 30th Street and re-enter "the" 30th Street Station. A long route home, perhaps, but it really won't take too terribly long and it's actually cheaper than taking a cab.