That is not what JPL does, despite its name. JPL makes what goes inside the nose cone of the rocket. The engine is that thing at the other end of the rocket.
As it turns out the thing at the top end has historically cost at least 10X what the engines cost. The lander that went to Mars is a 4 billion dollar device, the rocket that sent it there is about 200 million dollars.
JPL does the highest cost part of the mission.
The work of the Jet Propulsion Laboratory should be mainly that of the other end of the rocket, which ensures the delivery of the precious cargo, otherwise it would not be called that. As for how much the delivered freight costs. I think that the priority for profit is for private companies. Governments need to push technology forward, in this sense of the article, we need better engines. And since chemical ones are almost exhausted by development, they also have too little energy value. By now there should have been successfully introduced engines with a nuclear or thermonuclear energy source. If I don't count the problems in the 1960s, the many decades that have passed, progress should have led to technical challenges being overcome. I have the feeling that the state-owned companies of the USA do not seem to know that we are already in the 21st century, and their internal calendar has stopped at the beginning of the 1960s.