It does vary: a classic is LON/MAD. Head out and get a one way LHR/LCY to MAD - relatively cheap (c£69 when I was doing the route regularly last year); try the other way from MAD to LHR/LCY and it would be the Euro400 range and above.
I'm sure IB had something to do with it, but if I had to 'start' a journey in MAD it would always be a return or a one-way onto somewhere (like EDI) with a '23hr' connection so I could off-load in LHR.