I went from Spain 6 days and I had a great time. 2 days at Disneyland/DCA, 1 at Universal and the rest we saw Los Angeles/Hollywood.
We spent the nights at a hotel in Anaheim 5 minutes walking from Disneyland. We rent a car for the week to go to Los Angeles and Universal.
All the trip costed us around 1200 euros (park tickets, renting car, even souvenir and food included), the most expensive was the plane. The hotel was cheap and amazingly good and clean (I recommend it, look for Clarion Hotel Anaheim Resort in google. I think it was 140 euros 5 nights without breakfast, not that you need those).
On a 10 day trip I would add another day to Disneyland, and also go to Six Flag Magic Mountain and Knott's Berry Farm (somethign I really wanted to do when I wnet there and couldn't becuase of time). If you don't want to visit more parks, maybe do a trip with the rent car to Las Vegas or San Francisco, but that means being on the road for lots of hours.
We went in the middle of June, had great weather (Mornings were always cloudy though), and decided to go to the parks during weekdays so we didn't do many queues.
Hope this helps.