JWST is fundementally a different type of space telescope from Hubble, all of which contribute to its costing:
* It's being deployed at L2 Lagrange point instead of LEO - so the launch costs are even higher.
* HST had the insurance of knowing that repair was possible - JWST does not (both due to it's location and lack of capability) and so everything drags on even more to try to reduce risk
* JWST is technically more complicated. The thermal management system needed, and entire origami unfolding mechanism alone would make it more complicated and expensive. This doesn't even start looking at the optics.
* It's being deployed at L2 Lagrange point instead of LEO - so the launch costs are even higher.
* HST had the insurance of knowing that repair was possible - JWST does not (both due to it's location and lack of capability) and so everything drags on even more to try to reduce risk
* JWST is technically more complicated. The thermal management system needed, and entire origami unfolding mechanism alone would make it more complicated and expensive. This doesn't even start looking at the optics.