Cost is what PYC or the licensee/acquirer decides. Obviously...

Cost is what PYC or the licensee/acquirer decides. Obviously many factors come into play – government rules, potential market size, cost of development, COGs, whether there are any competitive therapies, level of therapeutic impact, which market you're selling into e.t.c., but it's long been argued that the main determinant in drug pricing is "what the market will bear".

In the case of Neuren's recently approved drug in Rett syndrome, another neurodevelopmental disorder with roughly half the potential market size of Phelan McDermid, the list price is US $21.10 per mL and patients take between 50mL-120mL per day. The quoted “average” price (that is, for a 27kg patient who is covered by Medicaid, which applies a mandatory discount of 23.1%), is US$375,000 per annum. For a privately insured patient weighing 50kg+, the price is ~US$924,000 p.a. Neuren’s licensee believes that roughly 60% of its patient population in the United States will be covered by the government (Medicaid) and the other 40% by private insurance, which in the US is predominantly employer-sponsored.

Neuren is testing a different (though related) drug in Phelan McDermid than the one approved for Rett syndrome. The company believes that this second drug is better.
Were Acadia (Neuren’s US licensee) to license this second drug and if it was approved, I would certainly expect that they would be pricing it at a level in excess of US$150,000 p.a.

By the way, if PYC's PPMO works as hoped in Phelan McDermid, I would expect it to have a higher level of therapeutic impact than Neuren's candidate. I guess time will tell!