The difference is just the Shuttle's External tank in 1995 cost 55 million each. not to mention the SRB reconditioning was almost as much as building new booste

The difference is just the Shuttle's External tank in 1995 cost 55 million each. not to mention the SRB reconditioning was almost as much as building new boosters. and those are only two costs of prepping a shuttle for each launch. not to mention the extensive processing workflow that takes place. so for the cost of 3 external tanks Nasa is getting a cargo resupply. Thats a bargain even with the current economics (which spacex is keen on shifting downward).