Things to account for:
a) Flights. No idea about school holidays but outside the school holidays do not pay more than £500 to LAX.
b) Car hire - budget £30 a day for a Luxury Saloon or a Mustang Cab. If it's more than this you've not looked hard enough for a deal.
c) Hotels - your average will be between £50 and £80 a night for the trip depending on your tastes and how close you need to be to city centres. Note average - San Fran for example will be three figures, middle of nowhere Motels will be much less. We now buy almost every hotel through Hotels.com - you get 10% Quidco and you get a credit for every night you stay, which after 10 is good for a 'free' night. They'll start adding up, we are usually redeeming several 'free' nights per trip. Hotels.com is usually as cheap as it gets anyway so the bonus points and cashback really add up.
d) Food - this is entirely personal. We are not that fussed so are happy enough with a $30-40 bill for two at the local Applebees grille...
e) Activities - how long is a piece of string? Our only real activity cost tends to be National Park fees these days. This year we did 9 national parks so bought an America the Beautiful pass for just $80.
We tend to spend much less than you'd expect as the focus on being sufficiently frugal that we enjoy the trip but the trips are still sustainable now they are an annual thing rather than a 'once' type trip where you'd be more tempted to go mad on best hotels ever and gourmet dining..
Also think about how you pay for the trip - I pay for everything on a credit card with some sort of introductory bonus now which usually gets me a couple of free nights at least on the trip. This time we stayed for free using Hilton points from a credit card signup bonus at the Waldorf Astoria Resort in Park City
Few trips before that we had 3 nights in a Sheraton in Vancouver for nothing using Starwood points transferred from an Amex signup bonus...