One Starship has close to the volume of the International Space Station. Instead of costing $150 billion and needing 60 launches to assemble, each Starship would cost 4000 to 8000 times less.
SpaceX will need to use about five launches of Super Heavy Starships to fully fuel a Starship in orbit. They will then send a fully fueled Starship to Mars. The Six Raptor engines in the Starship will take the Starship to Mars with about 100 tons of payload.
The Starship is only about $300000 worth of stainless steel and six Raptor engines. There is also the cost of life support, electronics and other systems. The main cost will be the Raptor engines. If the Raptor engines are twice as costly as the Merlin engines, then the Super Heavy Starship would cost about $200 million. The Starship would be about $40 million of the cost based on having about 20% of the engines. Mass production of Starships could bring the cost down to $20 million. The limitation on the cost is the future unit cost of Raptor engines.
Fuel could be produced on Mars to fly a Starship back to Earth. This will likely be a relatively rare occurrence. SpaceX will have many Super Heavy vehicles and Starships.
On Mars, a lower amount of refueling would allow Mars point to point travel and a habitat for 100 people with each Starship.