In my personal experience commuting by bike has not saved me any money. I'm one of those who can't stand it when the bike makes any noise at all, and I want it nicely tuned etc. So I find myself spending money and time tweaking the bike. There will be maintenance cost and consumables you may need to buy right away.

For example, I just got a 90s Cannondale mtb of craigslist for $40 (which I thought was a great deal), but I've thus far spent $200 on tires, tubes, handlebar grips, chain, lights, and new cables, and I already have all the tools grease and lube to keep it maintained.

You don't have to spend that much money on the bike, but you will most likely have to spend some (like a lock for campus parking).