A big share of the cost of Shuttle launches...
is the labor cost of thousands of employees. It literally takes a cast of thousands to prepare and launch a Shuttle. These costs are fairly fixed - NASA pays people pretty much the same whether they launch 1 flight a year or 10. That's why it's so hard nail down the costs per flight. Not that Shuttle parts are cheap, either, but labor costs really make the Shuttle so pricey.