This is a difficult question because I have not bought a 'bike' in many years, just parts, wheels and tires to keep the Bike of Theseus rolling.
I don't really track my mileage. Based on an approximate cost for a comparable bike plus regular maintenance, I would estimate owning a bike similar to mine would have cost about $3000 over the past 15 years, and I estimate 15-20k km of riding, so between 20 cents/km and 15 cents/km