I just sat down to do the math on the "couple hundred dollars" and I think that the figure is pretty optimistic. Unless you start by finding the deal of the century, it will cost more than that to get set up for full-time commuting, although occasional commuting could probably be done for less. Here's roughly what it cost me to get fully up and running. This did not happen all at once, but you should be prepared for it.

Bike - $0 (Trashpicked Schwinn World Sport frame)
Wheels - $70 (replace old steel-rims, better braking)
Derrailuers, crankset, brake levers - $10 (from another yard-sale bike, 105 w/Biopace chainrings)
Stem, bar - $40 (pure vanity. Could have used old stuff)
Paint - $15 (spray cans)
Freewheel - $25
Chain - $20
Pedals - $20
Bar tape - $15
Blinkie - $10
Rack - $40
Panniers - $90
Fenders - $45
Computer - $25
Pump - $25
Headlight - $90
Rain jacket - $32 (o2, ProPore)
Rain pants - $40
Reflective vest - $20
Tires - $25

Add this up and my "free" commuter cost me $592. Much of this was on the necessities of commuting, not the bike itself. If you don't intend to ride in the dark, your cost could be a bit lower.

I wear a pair of shorts and a Walmart CoolMax tshirt in the summer and just wear my casual work clothes in the colder weather. I keep couple of pairs of shoes under my desk. My commute is 5.5 miles and I only need to do a quick washup in the bathroom when I arrive at work during the summer months.