I recently rode from Bellingham down to Newburg, OR (just south of Portland) (Journal here
). I had great weather (I did it in late August) and enjoyed much of my ride in Washington. I didn't really follow either ACA route in Washington, riding through the islands, down the Hood Canal, over to the coast and then down to Oregon. The time I spent on the northern Oregon coast (Astoria to Tillamook) was nothing special, with the exception that the coast from Cannon Beach to Nahalam Bay was spectacular. I have no experience south of Tillamook, though I have heard it is beautiful.
I have also ridden from SF to LA many times as I live in SF and am from LA. The first ride I did, I averaged 70 mile days (Journal here
). It took me 11 days, though I had 2 rest days. It was hard and I was often very tired at the end of the day. On other rides, I averaged about 50 miles a day and enjoyed my ride far more (Journal here
To your questions:
1) 1800 miles in 21 days is too much, in my view. I aim for no more than 50/day knowing I could do 70+ if needed. This also leaves you no time for mechanical failures or spending time somewhere along the way. Also, riding to a strict schedule puts undue mental pressure on something that is supposed to be fun. Frankly, I'd plan to do less than more. I aim for no more than 250 miles/week.
2) I have never ridden the AIDS ride but I have driven Highway 1 when they were on it and it was a steady stream of bike riders in all modes of dress and distress. Frankly, unless you like being part of a larger spectacle, I'd avoid it as they will overwhelm the services along the road and road side. They don't close the road. In the past, they have headed inland instead of riding the Big Sur Coast. This makes sense as there isn't any camping place big enough to handle the hundreds of people on the ride on the coast.
3) I don't concern myself with costs when touring. But, if you use warmshowers and couchsurfing (far more hosts) and stay in hiker/biker sites, your overnight costs should be minimal. Also, there are many Subway sandwich shops along this route were for $7 you can get a day's worth of calories.
4) Don't miss Big Sur. As someone earlier said, it is only a problem during big storms (which we haven't had this year). If there is a slide before your trip, it might be a while before the road is reopened. Check various biking forums for local reports should this happen.
The further north you start, the more likely it is to rain part of the trip. Be sure to carry raingear.
Most of the ride is on roads with little to no shoulder. Take a rear view mirror and check it constantly. Be ready to pull off the road, if necessary.
Carry name cards that provide ways for people to get in touch with you or follow your ride. It is nice to hand something to someone you've made some contact with.
Take lots of photos.