Market has not priced it wrong - CNS drugs are tricky and they are approved till they are actually approved by FDA or other agencies.
At this point, Rett results are still optimism. Once the results are out and they show actual efficacy, market will give due valuation.
The Rett pricing at $200k to $600k is clearly very high - co will face serious pressure. Also that pricing may not work outside USA (check Bluebird gene therapy - they withdrew the approved drugs from EUrope as they are not able to get reimbursement)
So in my view - this stock has great potential but we need to see results of at least one phase 3 or pivotal trial that can help submission for drug approval - till that time it is castles in Air.