As a side note, considering the calorie costings above, my commute in is 13miles. According to strava I burnt 540 calories on the ride in today, considering a loaf of bread costs around £1.20 and contains around 1800 calories, the cost of my ride in was around 35p so 70p per day or £3.50 per week.
Now I'm making some (reasonable) assumptions here, namely that over the course of the year I will replace my chain twice (£15 each), my cassette once (£20), and my tyres (£80) once. Given that my daily train fare is £5.60 that equates to around a £950 a year saving, adjusting the working time for 5 weeks annual leave and bank holidays.
(((47 * 5) - 8 ) * 5.6) - (((47 * 5)-8) * 0.7) - 30 - 20 - 40 - 80