Tried booking the bike and two adults on for our usual Monday through to Friday of S100 week and SP wanted a whopping £287.38. Checked car/camper van and the same trip £205.  Queried why a bike should be £82 more and it's because the limited 5 day short break allowance for bikes have been sold.  Would not move when asked if they could be a little more accommodating.  

Ridden the island a thousand times, coming specifically for the racing and stopping in Castletown so that's Easyjet booked for £98 for the pair of us.