The new routemaster-style buses will cost £354,500 each.
But Transport for London claims that the final contract, signed with manufacturer Wrightbus, has saved millions of pounds for the taxpayer.
They say the contract "includes a competitive, fixed price deal for the vehicles - removing the risk and uncertainty of higher production and material costs, and inflation over the next four years".
TfL says the price is "broadly comparable with a standard hybrid double deck bus", with the difference "accounted for by the much higher specification of the new bus compared to a standard hybrid bus".
They also claim that the new bus is "the greenest diesel electric hybrid bus in the world".
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A lamp in the tower which houses Big Ben is to be switched off for the first time in more than 70 years.
Sir Bruce entertained audiences for more than seven decades.
Work in the tower will also pause over the bank holiday weekend.