How much would it cost to own an decent yacht?

Your questions are unanswerable without stating price range, boat length, layout preferences, etc. There are literally hundreds of threads on this topic if you search YF, in particular the General Yachting Forum. Once you have narrowed down your search, post more specific questions and wisdom will follow.

Carl's answer is about as good as it gets. LOTS of details required to know how to guess. Do you want a 4WD yacht? Do you need to transport stuff? (Using your car analogy.) How big? How fast? How far? How often? What waterways?

And also... yacht drivers don't just appear overnight. There's a significant learning curve, and it takes "a while" (!!!) to gain sufficient experience to be safe. (Remembering that you can't just pull over to the shoulder of the road and step out for a breath of fresh air.) So who will operate it? You, eventually? Hired crew, at first? Or...?

Another short answer to the "how much" question might be "at least twice as much as you pay for it." Or another semi-common: What you paid for it, plus at least $10% per year ever after. (The 10% is a common WAG, but it fluctuates wildly from year to year... and I'd guess 20% is likely closer.) And those are optimistic guesses. But remember, you gotta moor it somewhere, and that costs. You gotta have insurance. You gotta pay for fuel... and these don't sip diesel like an E220D. And so forth.

And unlike cars, with an oil change every several thousand miles and some tire replacements occasionally... big boats include lots of systems. A galley, with most everything in your kitchen at home. HVAC systems, the more the merrier as the boat size grows. Multiple engines, multiple electric generators, sometimes solar. Et cetera almost ad infinitum. Who will service all that stuff? You? Or hired labor, at $100-150/hour or more?

Look up some boats around here that might float your boat, and put those forward as possible targets... add WAY more details about how you might actually USE such a boat... and folks might be able to home in better with some maybe-more-useful feedback.

-Chris