Housing makes a huge difference, but there is also the cost of groceries, dining out, etc.

Basically the housing price difference can mean buying a nice house close to your job vs renting a room in a share-apartment.

Best of both worlds is to save in a high-cost area then move to a cheaper area.

Not sure what you're getting at. Housing is the main cost, and is drastically cheaper in Seattle. Food in Seattle is a bit more pricey.

Housing is just one component, there is a lot of other stuff that has equal price: if you order stuff from Amazon the price is the same, if you buy a new car the price is the same.

State and local taxes can make a significant difference for general goods, and especially car purchases.

Amazon also isn't a restaurant, and while they do sort of sell groceries through Whole Foods and Amazon Fresh, those are again priced locally.