This is a wild guess as I like to eat meat and fast food and spend much more. Mind you that this advice is coming from Mr. Cheapskate himself. As a result, I can in good faith revise my estimate based on Mr. Lear's advice. He's a literary type afterall and not very good with numbers in the first place. I can't tell you how nice it was to be able to have Guinness with dinner every night.
Transportation is big.
Thus I hope you now see the need for some sort of corporate sponsorship. Without it, you're forced with a bunch of unenviable options. You can drop your mileage and work which sort of goes against the goal of trying to be the best marathoner possible. If you don't pick up a job, you are forced to pick and choose your marathons, trying to find some obscure slow one where you can win a lot of cash. With corporate sponsorship, one could actually go to Chicago and worry about running as fast as possible.