They previously threw around a number of around $1M per flight, as mostly fuel costs.
Also, while 150t is the target payload capacity, the current test vehicles are closer to 50t in payload capacity, there are revisions in the pipeline based on data from these test flights which will bring it up to 150t.
To put this in perspective: at 150t/launch, if a launch is $1M, then for the cost of an SLS launch (at least $2B) Starship could launch 300,000 tons, about the mass of three Nimitz-class nuclear aircraft carriers.
None of the vehicles have demonstrated any payload capacity yet. 50 tons is the on-paper capacity only, and seems quite high given how little fuel is left when the bring an empty starship to orbital altitude. I assume that as the engines and launch procedures get more efficient, they will start being able to bring stuff to orbit (and quite a bit of stuff, too).
They've actually been having to dump propellant in order to more accurately test what a Starship in orbit would be like, given they're not flying with a payload that would consume that propellant on ascent, but that they still want to launch with a full tank.
The dumping of this excess propellant actually caused an explosion and loss of vehicle on the second test flight.
That's what they said about the second test flight (and the third), but the webcast recordings looked a lot more like fuel leaks to me, and that is in line with Starship and early Falcon's past issues. I'm going to press X to doubt that the dumping narrative is the truth, since a nice face-saving white lie is in every corporation's handbook.
Also, while 150t is the target payload capacity, the current test vehicles are closer to 50t in payload capacity, there are revisions in the pipeline based on data from these test flights which will bring it up to 150t.