http://www.space.com/astronotes/astronotes.html

The JWST was headed for a 2011 liftoff. However, project costs have
ballooned and now far exceed the program budget. The project is now
price tagged at $3.5 billion. These increases come from a combination of
additional required work, schedule slip, the delay in a firm decision to
accept the Ariane 5 launch vehicle offered by the European Space Agency,
and increased reserves to meet NASA standards.