Well, Containers are certainly cheaper when they are serverless, yes. But... there is no way PAD would work in a serverless environment; even if you could ship it as part of your container, there's no way it could run without a windows runtime and drivers and all the underlying dependencies of PAD.
Theoretically, you can run Selenium in a Serverless environment (this blog walks through it in a rather primitive use case), and PAD runs Selenium, so I guess theoretically it might be possible in some fashion... but really, whatever money you're saving that way has got to be eclipsed by the work it would take to make it happen.
Maybe just spin down your VMs at all times when not in use to keep that cost in check.