Since you asked about old bikes, here’s a breakdown for a classic old beater, a '99 Kymco Jockey 125:
Gas about $200 a week—that’s about 110-30km or 20 km/l (def worse than a newer fuel injected bike, but still ok). ~$9600 p/yr
Oil change $150 every 1000 km and $250 (w gear oil) every 3000 km. This is the cheapest regular Kymco brand oil—you don’t need anything better for this type of scoot. $1200 p/yr
Shocks, belt change, shock oil change, battery, brakes, carb, misc. stuff etc. $2000-3000 per year.
Tires $700 each. About once a year for rear. Front lasts twice as long. $1000 p/yr
Total: $14,400 + insurance ~ $15,500
All bets off if something major blows, but you can’t predict when that will happen and the worst possible expense would be new cylinder, which for this kind of bike would be about $7,000. So far I’ve racked up 90,000 km on it with no major blow outs, and since it was a second hand bike, the total is probably more than 100k. Just make sure to change the oil on time—nothing kills an old bike faster (or a new one for that matter). That means keeping the odometer cable working so you can keep track of when to change (many bikes have a little red/green flip counter)—these cable tend to snap easily.
Almost 16k doesn’t sound super cheap compared to a $10k bargain beater you may have just picked up, but to put that in perspective, if I took the MRT at $25 each way 5 day a week, it would be $12,000 per year alone, and I’d still be stuck unable to quickly do my shopping, go eat, go here there, etc., which would all easily rack up to more with extra fares and cabs.
Conclusion: Worth it.