Here's how the Tokyo math works out:
Right now a one day ticket in Tokyo is 8900 JPY or just under 65 USD. If you go 3 days, Tokyo Disney tickets will cost just over 190 USD vs $360 for Disneyland. Four nights at the Sheraton Grande which I consider to be the best Disney Official Hotel starts at 66K JPY or just over 470 USD in January. The Westin which I consider to be the best non-Disney hotel near Disneyland starts at $1052. If you compare the Disneyland Hotel to Tokyo Disneyland Hotel or the Grand Californian to the Miracosta, the price difference will often be around 150 USD a night as well. Flights from San Francisco to Tokyo will likely cost between $900 and $1100 compared to around $200 to get to Disneyland round trip including ground transportation.
Since the price is basically the same, once you factor in food, merchandise and
Genie+ or Premier Access, it can be slightly cheaper to visit Tokyo Disneyland if you pick the right dates, especially for one person. With a family, you really have to spend a lot on these extras for it to make up the difference in airfare but it is theoretically possible for the Tokyo trip to be cheaper.
For most people living on the West Coast of the US, the trip to Tokyo will likely not end up cheaper unless they already had a reason to be in Asia anyway and aren't factoring the full cost of airfare in this comparison. I personally am willing to stay at a lower quality hotel when in the US compared to in Japan which means I usually pay a similar amount per night. Additionally, most Americans visiting Tokyo Disney will likely turn it into a bigger trip with lots of additional expenses.