I've had about 9 'once in a lifetime' visits to
Disneyland now.

Hopefully our 'once in a lifetime' visit to WDW last year will continue along the same path.
But back to the question at hand: Last year, DH, DS12, DS8 and I spent 6 weeks and went to New York, Washington DC, WDW (12 nights), Disneyland (15 nights) and it cost about $30,000.
I am a bargain hunter and used sites like Expedia ruthlessly. I refused to pay the outrageous prices for accommodation in NY and DC, so we stayed in neighbouring states! We spent US$84 total for the 4 us on a Greyhound bus from NJ to DC. Great way to see the countryside and no airport hassles.
Instead of NYC itself we stayed in New Jersey and commuted in via train each day. It only took about 15 minutes and cost approx $10/day for the 4 of us. The hotel we stayed in was lovely and only cost about $110 per night, including a delicious breakfast each morning.
Instead of staying in Washington DC we stayed in neighbouring Virgina (gorgeous!) in a beautiful hotel for around $90/night. It included a free transfer to the train station each day and the train trip only took about 15 minutes.
With the money we saved on those hotels I decided to splurge and stay at the Disneyland hotel for our final 4 nights. Our original price was about $1900AUD for a basic room with a view of the carpark. 2 days before checking in I rechecked the price on their website and managed to get it down to about $900. I can say, without a doubt, that the DL Hotel was/is one of the worst hotels I have ever stayed in. Ugh! It wasn't even worth $100 a night. Never again!
I bought 14 day DL tickets from Expedia which cost about $900. Our Qantas airfares from Brisbane-NY-LAX-BNE were a bargain at $4564. We probably spent about another $800-$900 on internal airfares DC-Orlando-NY. We struck it lucky and got free dining at WDW, saving us about $2000.
We stayed at Paradise Pier because I got an Expedia special of about $129/night. It was an okay hotel. Our best Anaheim hotel was Desert Palms - free breakfast, great price, staff, room (suite), location. Will definitely stay there again.
The whole trip was worth every cent (well, except the DL hotel). I'm now scanning the internet looking for bargains in the hopes we can go to at least DL, if not WDW also, in December.