For me, 0% as I cycle everyday. I wouldn't include parts for my bike, because even if I didn't commute I'd ride as it's a hobby anyway. Wear on chain/cassette etc. is probably a couple of hundred per year, so 0.11111111111111% - my wife's car is a little 3cyl C1, so uses about a tank a month? So 50 euros a month, 600 a year, or 0.789% of her salary.
You'd probably want to factor in 500 a year in car costs though, and I haven't included car insurance, but that's just the general cost of the car and not directly linked to commuting.
I used to work in London, live in Old Hatfield and I'd spent most my wages on a season ticket and it was brutal. Not only brutal seeing the money go for just getting to work, but also the utter **** of having to be on a train for 2-3hrs a day.