The one time I went to Herne Hill Cycles, it was to enquire about the cycle to work scheme. At that time I was working at Monument, taking the train to Elephant and walking the rest. My train ticket was less than a tenner per week.
The guy was used to selling bikes to people buying travelcards at £30. He then really stuck the arm in, telling me I needed two locks, paniers, pump, trip computer, waterproofs etc. And basically got the price up to £100 per month, telling me about the tax saving. The accessories cost as much as the bicycle.
I told him it was far too expensive, he looked puzzled and I just left it there.
So, if you buy in HH Cycles, make sure he doesn't try to sell you half the shop.