Sorry but the OP states it's over budget and overdue. Well if you look at the original budget & deadline yes this is correct, however, subsequently the scope of the project has been massively increased which consequently increased the budget and time scale. Its not due to fly until 2018 and has still cost less than the Hubble.
There are several things to note here. First, the cost of Hubble included six Space Shuttle launches and 24 years of operation. Second, The JWST (James Webb Space Telescope) is eight years behind schedule. Third, massive increase in scope of a federal project is a common ploy for siphoning more funds. Maybe nothing untoward happened with the changing of JWST's scope, but it's an easy thing for a bribe to arrange. And the project went on for five more years as a result of this changing of scope.