I only commented on here occassionally prior to my road trip, but found this site incredibly useful, so thought I would post a little about my road trip experience.
On 11 May picked up a Dodge Magnum in San Fran (free upgrade, stationwagon, fun looking but not a great car). Stayed at Santa Cruz.
12 May - Monterey Aquarium, Carmel Mission, stayed in Seaside.
13 May - Drove Pacific Coast Highway, stayed with family in Newport Beach, LA.
14 May - Disneyland - interesting, but one day is more than enough.
15 May - Huntington Gardens, Pasadena - less touristy/tacky than Hearst Castle.
16 May - Drove to Death Valley, camped at Furnace Creek having been assured it would cool down at night - it did not.
17 May - Las Vegas
18 May - Hoover Dam, drove around Lake Mead towards Zion NP. Stayed in Hurricane.
19 May - Explored Zion NP, climbed Angels Landing - amazing. Camped in the park.
20 May - Drove to Bryce NP, explored the park.
21 May - Drove to North Rim Grand Canyon, then on to Page, AZ for the night.
22 May - Drove towards Flagstaff, stopped in at Wupataki NP.
23 May - Drove through Sedona, down to Montezuma Castle, then to Walnut Canyon, stayed in Holbrook.
24 May - Explored Petrified Forest, drove to Albequrque, NM for the night.
25 May - Headed to Bandiler NP, camped there.
26 May - Explored Los Alamos, drove to Taos, NM and camped there.
27 May - Drove to Colorado Springs, stopping by Great Sand Dunes NP.
28 May - Went up Pikes Peak on cog railway, camped in Rocky Mountain NP.
29 May - Explored Rocky Mountain NP, stayed on western side.
30 May - Drove to Ouray, camped.
31 May - Drove to Mesa Verde, camped and explored.
1 June - Drove to Moab.
2 June - Briefly visited Canyonlands, Arches, stayed near Grand Junction, CO.
3 June - Drove to Green River, WY via Dinosaur NP.
4 June - day of rest - relaxed and watched TV.
5 June - Drove to Grand Tetons, camped and got very wet.
6 June - Drove to Yellowstone, with looming poor weather forecast and wet camping gear got a room in a lodge inside the park.
7 June - Snowed in - stuck at Old Faithful for most of the day - finally roads opened and drove to Cody hoping for better weather.
8 June - Buffalo Bill Museum - weather still poor, so drove to Little Bighorn, MO, camped nearby in Hardin.
9 June - Beartooth Highway still closed, so head back into Yellowstone by north entrance, camp there.
10 June - explore Yellowstone.
11 June - head down to Grand Tetons again and camp there.
12 June - Drive down to Salt Lake City, via Idaho, and Antelope Island in the Salt Lake - swam in it, which is both cool and gross at the same time.
13 June - Drove to Lake Tahoe.
14 June - Drove across Tioga Pass, explored Yosemite, camped nearby.
15 June - Walked up to Nevada Falls, saw other parts of Yosemite. Drove back to San Fran, returned rental car.
We then had a week with family in the SF area, and had already spent two weeks on East Coast in Wash, Penn, and NYC.
We averaged about $55 per night (incl tax) when we stayed in motels, no where was booked in advance, although we made sure we got to whereever we were heading by about 6pm. When possible we made use of the motel coupon books, which had some good savings. Camping was suprisingly expensive, it is difficult to justify paying $20 per night for a site with a long-drop toilet, when you can get a comfortable motel for $40. Having said that, camping was a great experience, once we got used to the whole food security thing.
We had a cooler which we put lunch food, snacks and drinks in - this also saved a lot of money. If we were staying at a motel, we ate out for dinner and this averaged to around $20 for 2 of us - varying between $12 for McDs and $30 for a 'nice' meal.
Overall - we averaged around $100 per day for accomodation, petrol and food for 2. We purchased a NP annual pass at the start, and certainly got our moneys worth from it.
We probably did not take the most sensible route, in fact almost certainly did not, but we were kind of making it up as we went along. We had a general idea of what we wanted to cover, but changed the plan depending on weather, how interesting or uninteresting we found the area. This worked really well for us. We drove A LOT, and by the end of it were really sick of driving, but I'm not sure what we would of cut out to try and avoid that. We also fitted in a lot of walking/museums/other attractions. It was an amazing trip - the US is a beautiful country.
Well that is my two cents anyway - I hope it is useful to someone.