I don't know the cost in US, but in my country Euro prices are: highway at 6 million/km, rail at 10 million, viaducts over 15 and tunnels over 20 million. The elevated rail would be at least 50% more expensive, probably more than double because in case of viaducts there is no restriction on what is below the construction and you can put pylons anywhere, if it is above and existing highway there are many limitations.

So it is doable, maybe a good idea if you have the money, but I heard California is burning money like there is no tomorrow and there are no funds for such a project (maybe not true, it's just what I heard).