There is no doubt that the automobiles have changed thru the years. As far as the prices - it depends on what you buy.If everyone started using public transportation, riding a motorcycle, carpooling, bicycling, or walking - the cost of a car would go down - supply and demand principle.
Ahh, that's a cute idea. Over 80% of America doesn't have any type of public transportation whatsoever. The areas that do are usually in large cities and in cities like Detroit, no one rides public transportation for fear of being assulted, raped or robbed. Motorcycles are impratical. They only seat 2 at most, you get wet when it rains, you can't ride them in snow, and they are dangerous. You are something like 10 times more likely to have life threatning injuries after a minor accident than you would in a car. Bicycling and Walking are nice ideas, if you live less than a mile from work, school, the grocery store, etc. In many areas of America this isn't the case for most of any rural or suburban population. Carpooling sounds like a good idea, until you realize that it severly cuts your freedom to go where you want, when you want because everything is too far away to walk or bicycle.
Your ideas are nice, for Europe. They don't work so well here where we don't integrate housing, shoping, and jobs into the same areas a lot of the time. Or where people work 20, 30, 50 miles from where they live.