I see this comment from a lot of east-coasters and midwesterners. I grew up in the midwest, in the Chicago area, and thought the same way when I lived there.
I moved out to the west coast 30-some years ago, and discovered an interesting phenomenon. Used cars out here keep their value, and go for an incredible amount of money! It makes sense in a way. With no salt on the roads to corrode them, and mild weather to keep them in good shape, used cars here should be a little higher than elsewhere. But the extent to which it's true is amazing.
I generally use the following metric. I pro-rate a used car value based on a 150K mile lifetime. Yes, a good car will last longer, but I use 150K, because after that maintenance costs start to eat up the advantage you have in not paying for depreciation. Anyway, by my metric, your example used car with 50K miles should sell for 2/3 the price of new. Any more than that, and you're paying more per mile for the used car than you will for the new.
And you know what? In 30 years of living out here, by my metric, new cars have always been cheaper per mile than used. That is to say, in California, your 50K used car will sell for 80% or more of the new price, rather than 67%.
LisaH lives in the SF Bay area, so I'd expect that she'd find the same phenomenon. So, while I would have agreed with you when I lived in the midwest, Ron, I'm not sure it's good advice for Lisa.
BTW, I don't have a decent answer to the original question. Personally, I buy new and keep the car for at least 150K miles. Currently DW & I own two cars, with 100K and 200K miles respectively. If your usage pattern is like that, then buying is most cost effective. If you want a new car every 3-5 years, leasing can make sense. YMMV.
-Bob