Ooh, yes, nice one. Three Bridges to/from Victoria: £8.90. You do have to buy a ticket that permits a break of journey, although Wikipedia notes that, now that stations are all non-smoking (with a single exception), barrier staff (if there are any) are obliged to let you 'leave for a cigarette'. Caveat Wikipedior and all that.

There's a similar trick you can do at Stansted, using Audley End (£14.90 instead of £19.00). However, as there are direct trains from Audley End to Liverpool Street, it's a more difficult one to argue (it is a valid route, though - specifically stated in the routeing guide)