JWST talking points
From what I know, the "talking points" (from NASAwatch) are substantially accurate.
In particular a lot of the additional cost is due to:
1) Full Cost Accounting changing how NASA center costs is charged to mission (think of it as internal overhead)
2) Delay in choosing launcher. Politics of accepting it had to be sent on Ariane 5 were slow in coming in to accord with reality...
3) Inability to meet short term increased costs means schedule must be stretched, which costs more in total (it always does) then it would to just surge one year costs, but NASA budget doesn't have that flexibility.
rest is "normal" cost overruns ($200 million or so), inflation, added contingency funds (contingency funds scale non-linearly with cost, based on experience, as I understand it), and contractor cost increases.
Crazy way of doing things, but not entirely NASA's fault and only to a small extent the sciencists and engineers' fault...