********** AHHHHHHHHHHHHHHHHHHHHHHHHHHHHHH*************
*************** STOP!! *******************************
Forget all that! 75 cents will take you from Santa Monica to the airport. Take the Big Blue Bus #3 (marked "Airport" or "LAX") from either Wilshire or Lincoln. It's only 75 cents!!! Go to terminal #1 (or any terminal) information desk and take the Disneyland Shutle for $20 bucks each way. Leaves every hour before 10.30AM and after that every half hour up until 7.30PM. Better to book 48 hours in advance to secure your passage. I forgot the website address. It's something like the "Anaheim express" or some such name. You can google all this and look on the disneyland web site for more info.
This is an old thread! I don't know how you picked up on it today, but I think the OP has long been to DLR by now!
By the way, you and I had the same idea - get to LAX and take the Disneyland Express from there! So I beat you to it long ago!

The only difference is, I was taking into account that many people don't like taking buses, and prefer the privacy, convenience and more direct route of a taxi or a shuttle!! Obviously buses are cheaper, but they are a more circuitous way of getting to LAX and you have longer waits and drive times, etc. But, either way, getting to LAX was the goal!
In any case, since the OP never followed up with us, we have no clue as to whether or not they rented a car, took a bus, a taxi, a shuttle, a helicopter or if they even went to DLR after all!
Oh, by the way, I thought of taking the Disneyland Express myself, but if I were to go on a solo day trip, I would want something that was not so limited time-wise. I would want to get there at park opening and stay closer to closing than when the Express leaves. That is another thing to be considered, and another point in favor of taxis and shuttles and buses, if one is so inclined. You can arrive and leave when you want rather than when the Express leaves.