I've already got the bike sitting in the garage, and so am paying all the costs irrespective of whether I use it or not. Inurance + Tax + MOT etc, and when it comes to a service that's pretty much a set fee irrespective of mileage covered - and will actually cost me more in stuffed battery and stiff pivots if the bike has covered zero miles since the previous service.
Over the years the only "consumables" other than petrol and the ones always on a service (oil/brake fluid/etc) have been tyres and brake pads - and when you break a tyre cost down over 15,000-20,000 miles it's practically negligible.
Edit: assuming my most excessive wear, I've just calculated tyre cost at £3.96 for the return Bristol trip. This bumps the total price up to £31.96, which is 23.4% of train fare. However tyre wear is got on the twisty country roads, not sitting at a steady 70mph on a motorway so £3.96 really is a stupidly high inflated price.