The cost of an expendable third stage is surely more than made up for the brute force cost savings of being able to launch 150T to LEO for the cost of fuel and a car wash.
I think it depends on the situation. It is not yet clear if 150T to LEO is real (and even if it is, you're likely sharing it with a bunch of other customers who might not want to fly with another rocket motor unless it has gone through quite a bit of vetting and paperwork; Shuttle banned the use of solid rocket motor 3rd stages after the Challenger accident) and it is not yet clear what the cost for a typical customer is going to be. Rocket engines designed to start in a vacuum are more difficult and expensive to develop and test than engines designed to operate closer to STP.