Saw these in Boots today and found it online as well.
Works out at £1.53 each.
Amazon are currently selling 8 for £18.39
Boots have also got a couple of high end Oral-B Toothbrushes at half price:
Triumph 9500 £53.82
Triumph with SmartGuide £68.40
Sorry I don't know how long the offer is on for though.