I'm guessing if you can't even get any at on-demand pricing, the demand is high, the supply is too low, so the only way to get access is to pay through the roof.
I'm wondering if the Cloud providers are seeing cryptocoin mining as the primary workload at the moment and don't want to be left with a ton of inventory the next time there is a difficulty increment and clients move on.
You have to request access to on demand P* instances so I suspect some people can't get as much allocation as they need and end up turning to the spot market.