At its simplest, commuting bike cost £265, to which I have added bar tape, brake blocks, new sprocket, tyres and tubes costing about £120 in total. I commute just over 16 miles a day, at least 230 days a year and I've had the bike for 2 years and 2 months. I make that a bit under 5p per mile, not including the extra miles done at weekends.
I'm glad the trek singlespeed is proving economical; I probably managed about 90p a mile on it!