Note though that faster travel is only going to cost even more work. With a larger area to cover, the lack of detail is going to stand out like a sore thumb. You've got a bigger area to cover, and players are more easily able to travel to the edge of the world.
If you want to save time and work, start small, and make that area seem big. You can always expand from there.
1) a metropolitan area comprises the game world. like the most recent versions of Fallout, only larger.
2) multiple unique areas in a region (i.e. west coast) comprise the game world, and are linked together through virtual 'fast' travel.
I think a single metropolitan area (with some surroundings) is the best way to go, then. If you want to tackle the entire west coast on a limited budget, it's going to be empty, with gaping holes, and any metropolitan area is going to seem small.
Better start with a single area. If you want a big one, I'd go with the Bay Area. It's classic Fallout territory, but it's not quite a single city. There's room for variation, and also lots of room to expand, if you want to tackle more metropolitan areas later. (LA seems like an obvious one.) And then slowly have the world grow to cover the entire west coast, if the game is successful enough.