Why is it so expensive to question patent's validity? It feels that if it was cheaper to issue challenges to obvious patents, it would be better for the whole ecosystem.

Tim O'Reilly launched BountyQuest a while ago http://oreilly.com/pub/a/oreilly/ask_tim/2003/bountyquest_10... but that died off, not sure a for-profit organization can be in the business of challenging dubious patents for a long time.