I actually worked this out when I was travelling back from London. The cost of the marathon for me was:
- £35 entry
- £240 trainers (x3 pairs on the go)
- £100 clothes (tights, tshirts, hi viz gear)
- £150 nutrition (gels/energy drinks/protein shakes)
- £170 Travel to London (2 return tickets)
- £60 Stuff from the Expo
- £120 Eating in London (for 2 people, 3 days)
- £15 Travel through London.
- £0 - Accommodation in london - crashed on cousin's boat.
= £900.
Admittedly - some of the stuff are for use after the event as well and I consider to be a "long term" purchase (e.g. part of the box of gels that I still have left, winter running tights etc) but it's good to have an idea of it before you start.
Entry is cheap but you need to factor in the likelihood of getting in. I waited 5 years for my ballot place.