In a universal donor scenario, the T-cells are engineered, error corrected and tested once. From there, they can be multiplied pretty cheaply. Most of the cost is in the initial engineering.
In a patient cell scenario, each individual patient's cells must be engineered, tested and delivered. This is obviously much more expensive.
A very crude computer science analogy would be the difference between scaling up a load balanced application relative to adding a new server from bare metal without automated scripts.
Why the treatment would cost half a millions dollars at the market is outside of my purview, so I'm eager to get an explanation as well.