Motorcycle commuting can be cheap, but it's usually expensive. Worth it or not is up to you - I don't regret spending a few years almost entirely on motorcycle in New Mexico, but it was expensive.
If you replace your car with a small engined, inexpensive motorcycle (250cc or so, maybe an older 500), you might come out ahead.
Otherwise, you won't.
- Tires: $300-$400 every 6k-12k miles, depending on tires, riding style, etc. I'm ignoring really sticky tires, which might only last a thousand miles or less of hard riding (which is not commuting).
- Gas: A higher power motorcycle won't outdo an efficient car for fuel economy. I was happy to see mid-30s.
- Maintenance: Most motorcycles require regular maintenance/valve checks/etc. Depending on the bike, this can be cheap and easy, or can be an expensive pain in the rear. I was spending about $1000/yr on valve maintenance alone (putting 14k miles/yr on does that).
- Gear: Figure a few hundred for initial gear purchases (minimum), then probably $200-$300 in gear replacement as things wear out, which they will if you're riding a lot.
- Farkles: Yes, you will tweak your motorcycle. A few hundred a year goes to that, too.
- You ride a lot more than you drive. This is the big killer for cost. If you replace a car with a motorcycle, and only use it to get around town, you might come out ahead. As soon as you start touring and doing longer rides on the weekend, and doing things with the motorcycle that you wouldn't do with a car, your cost savings go right out the window (off the tank?).
I don't regret riding around the southwest on a motorcycle, especially once my now-wife started to ride. But I don't pretend it was a cheap way to get around. On the other hand, riding something that gives you the power-to-weight ratio of a supercar on a daily basis has a nice way of making fast cars look slow and silly.
And I totally look forward to riding more in the future when I'm not in the rain forest parking lot of the northwest, but I don't pretend it's a cheap way to get around. Though, hopefully, it'll be a little bit more dirtbike in the future.