We just did this last week. The transportation cost was way more expensive than we were anticipating, $88.50+ $20 tip from the Queen Mary to Universal Studios then $94.93+$20 tip on the return. That was the absolute cheapest I could get it for. I kept waiting for prices to drop but they hovered there or went to $120-$140 at times. Those were both on Lyft, which ended up being a lot cheaper than Uber when we checked.
We needed an XL because we had 5 people, so that added into the cost but we were not anticipating it to be so expensive.
We had so much fun, but yeah - transportation costs are no joke. If you need a regular UberX, I would budget for $80-$100/one way not including tip since you'll be going from Disneyland. Check both Lyft and Uber for best price.
We went down to Disneyland from the Queen Mary and it was $84.50+tip. So crazy!