I've been slowly getting into bicycle commuting as well over the last few weeks. So far, in a good week, I'll bike 3 out of 5 days. I'd like to get that up to 5 out of 5.
I've found that while I have been hesitant to spend money on this endeavor ("what if I don't end up sticking with it?"), there have been a few purchases that have made it that much easier for me to take the bike (and harder to find excuses not to take it). Those were: lights, rear rack, pannier/trunk bag, fenders, mirror, retro-reflective ankle straps, and a synthetic base layer. A couple more purchases (mostly visibility/safety related as it is going to be darker a lot more in the next few months), and I hope I'll be set for a good long while: high-visibility jacket with retro-reflective elements on front, back, sleeves, and cuffs (important for cars to be able to notice hand signals at night), and some retro-reflective tape to put in various places on the frame. I may also upgrade my lock, as the one I bought several years ago with the bike is kind of crappy. If I really stick with it through the depths of winter, I might buy studded tires depending on how well-plowed the local bike path is (I've never had reason to notice until now).
My commute is 6 miles one-way, so it depends on how you calculate the cost of driving, but if you assume the standard reimbursement rate of around $0.50 per mile, I'm saving $6 a day by bicycle commuting. If I biked every day, I'd save on the order of $1500 a year. I'll probably end up saving even more than that because I'm more likely to bring my lunch or eat in the less expensive cafeteria at work when I bike, vs. driving somewhere to buy lunch. (Facepunch for that weakness, I know.)
It's nice to know there are others out there just getting started, and also braving the fall/winter elements! :)