$15,000 might be enough to get you through 5 months in the states, if you budget carefully and don’t eat out very often. If you are going to buy a car, I would add another $5000 for that expense. And beware of gas prices. They vary from state to state, region to region, even town to town. And fuel prices are going UP again. Be aware that prices of everything on the west coast (except perhaps produce) will be higher than other parts of the country.

Edit: Consider buying a tent that will sleep all 3 of you, and take advantage of some of the state parks across the country. You can pay a small fee for a campsite, and most state parks will allow you to camp for a 2 week maximum. For example, the Grand Canyon has campground, and one could easily spend several days there hiking, sightseeing, etc. Many of our national parks and state parks have campgrounds and camping facilities.

Camped on the rim of the Grand Canyon 4 nights, camped Yosemite Park for 2 nights.

By the way: There is absolutely no way you could get a car for 400-1000 that would make the kind of trip you are talking about. I am estimating at least $3000-5000 for something you can trust to travel cross country in!!

The first answer mentions couch surfer. Another organization is SERVAS, which is a global network of travelers & hosts who have passed a background check:

You can save on accomodations by using hostels, camping, and staying with hosts but you will probably have to stay in hotels some of the time. You can get hotel rooms with two beds and add a rollaway bed to stay all in one room, but budget a minimum of $100 per night for the times you will need a real hotel.

You can save by purchasing food in grocery stores, but your trip can’t be completely restaurant-free. Budget $50 >> per person per day

Luckily for you both of the above are worth more then the dollar so thats good.

Do you already have plane tickets? if not they are probably worth about 1200 round trip. I just got back from a trip italy and that was about what it cost.

As for a car its going to be hard to buy one legally without having citizenship, but im sure you can find someone to buy a cheap car from try craigslist.org and pick the state and city where you’ll be staying. Under cars for sale you can find the best deals and hopefully youll be able to get someone to sell you one easy. A cheap car would be about 400-1000 dollars depending on what your looking for. What might be even nicer for you guys is buying a cheap trailer (caravan) to get around with (saves money on hotels etc.) Most cheap hotels (without cockroaches) are about $50 a night. And you can probably find a few places to camp without having to pay.

As far as living, have you ever heard of couchsurfing.com? If not you should check it out. Thats what I used for 5 months while I was in europe and I met a lot of really cool people. The only thing is it cost like 20 dollars to sign up but its worth it in the end.

As far as food the best way to go is buying food per week and making it yourself (soups, sandwitches, etc) and that will cost about 100- 250 a week for all 3 of you. Do NOT go out to eat often! your money will be gone quick even at fast food restraunts.

Most amusement parks and such will cost quite a bit per visit. for example universal studios and disneyland will cost about $60 per person, but most places you can find good deals online just check out everything you can before you come over.

Basically you could survive here on about 2000 each for that long but that means you dont get to visit touristy things. that if you use couchsurfing, get a car or caravan to sleep in everyday, and just go grocery shopping every week for food.

But if you want to go all out you’ll proably want to bring about 5000-8000 each.
Hope I helped, if you need any more help let me know