I don't think I've bought anything I really needed for commuting since my first year, except of course replacing things like tires and brake pads that wear out. I still upgrade things here and there. Most recently I got a new fork to convert my Kona Jake to use a front disc brake. I'm contemplating a lighting upgrade next fall. I've bought a couple of new bikes this year, but they aren't for commuting.

So obviously there's a big disconnect between the first year cost of commuting and the ongoing cost of commuting.

Taking a guess at ongoing annual costs:

Tires: $150
Brake pads: $20
Cables: $30
Bar tape: $10
Chain: $30

These may be a little high, but I'm not counting things like cloithes, cassettes and chainrings that eventually wear out but not every year. It's probably less than $250 a year on average. I could obviously bring it down if I wanted to use cheaper tires or deal with sloppy shifting for longer.