The SpaceX Starship will have six Raptor engines but will still be larger and cheaper than the external fuel tanks of the Space Shuttle.
Elon Musk has a goal of building Starships for $5 million. Elon Musk has indicated that one of the reasons for switching to stainless steel for the SpaceX Super Heavy Starship was to use $3 per kilogram of steel instead of $200 per kilogram for carbon composite. SpaceX will save $10-24 million on cost of materials. If the Raptor and Merlin engines had similar cost then each Starship upper stage should be about half of the cost of a Falcon 9. A Falcon 9 combined upper and lower stage) costs about $30-40 million to build. This would mean that each Starship should currently cost about $15-20 million to build.
A lightweight external space shuttle fuel tank, ET 94, was built at a cost of $75 million. This would be about $140 million in todays dollars after adjusting for inflation. The lightweight tank was intended to pull the shuttle into low-earth orbit. This version was cheaper to build than the super lightweight tanks, which were able to carry more cargo to the International Space Station.
If SpaceX only achieves part of the cost savings that Elon Musk is targeting and reaches a $14 million cost then they would be ten times cheaper than a Shuttle fuel tank. If the $5 million cost is reached then the SpaceX Starship will be nearly 30 times cheaper than a Shuttle external fuel tank.
The lightweight fuel tank weighs 66,000 pounds. The Standard weight fuel tank weighed about 77,000-80,000 pounds.
The Space shuttle external fuel tank specifications were:
Height 46.9 meter (153.8 ft)
Diameter 8.4 meters (27.6 ft)
The SpaceX Starship upper stage dimensions:
Height 160-foot-tall (50 meters),
Diameter 30-foot (9 meter)