Not sure ... My KHS cost me $10 in 2001, I've spent maybe $36 on tires and tubes (I had a couple spare tires and tubes when I bought it)
A rear rack ... No ... I dug down to find an electrical conduit to pay for that ... Maybe $55 in upgrades, $24 for brake pads ...
I've put an average of roughly 4,000 miles a year on it ... so call it 68,000 miles ... with an investment of roughly $125 ...
My calculator says $0.0018 a mile.

(Of course after the new chain, cables, grips, and a couple other minor things, the cost per mile might go up a whole penny a mile. )