It's £14.00 return on a Terravision bus from Liverpool St. station, £7.00 return for a child.
A girl I know and her three friends went by black cab, they negotiated a fixed price of £100.00 from Rotherhithe, they each had one medium to large case and a smaller carry-on case, she said that the journey took fifty five minutes and in her opinion was well worth £25.00 each as it was door to door instead of cab/train to Liverpool St. then carry luggage to train/bus.
TO Luton you take the bus either from Victoria or from Gloucester Place near Baker Street. It's very quick and inexpensive and takes you right to the door of the airport (I've done the train from E&C but the hassle at Luton Station and the tansfer to the shuttle is a real bore). For Stanstead I've only taken the train from Liverpoool Street - it's a bummer it's so far from our part of London.
I'm not a great traveller, so I like to drive to The Harwood B&B in Gt Dunmow, park the car, have a (vege) curry across the road, get some sleep, then get a lift to & from the airport from Ian.
It's just so much easier.
Tell them Graham sent you.