ALL shuttle missions were more expensive than launching 3 separate expendable missions. (on top of the launch costs themselves, the payload had to be 'upgraded' to human safety requirements. So things like deployment squibs had to be made triple redundant (instead of double) and had to be completely disconnected from a power source until released from the shuttle bay. (and of course, that power connection system had to be triple redundant... the complexity fed on itself) and that's before you get to the paperwork and the safety reviews... And you had to staff up three launch teams, instead of two... you needed a full team at Johnson in addition to the Cape and your own site. And the launch teams needed to be onsite for the full shuttle mission and rehearsals, and jerked around by the incessant shuttle schedule slips. The costs just kept piling up. )