Welcome to the RTA Forum!

A very rough rule of thumb is $100 per day for a roadtrip, but there are so many variables that even that doesn't do a great job of telling the whole story. The reality is that it comes down to a whole number of factors, including what you'll be driving, what sorts of places you want to stay in, what you plan to do for food, and of course the length of time you plan to be on the road.

As far as the route itself, there are several thousand posts and threads already on this forum about trips along the California coast. If you need help starting out, I would highly recommend you simply take some time browsing and using the forum's search functions to see the many other ideas that people have already shared.