Honestly I don't even think that you (or the service you publish on) should even ask for money at all when delivering a demo. It should be free without "pay what you want" setting the implication that you should pay something. put up a donate button if you want donations.
Honestly, I don't think you understand what a developer goes through when creating their games. The art that's behind it, the long hours poured over for some random person on the internet trying to tell them how THEY should run THEIR GAME? How about this, you try and support a project by yourself without the option of letting those who'd like to support it (which is easily negatable by clicking the download button regardless) and see where that gets you. There are TONS of demos on Steam and many other platforms that people pay for. It supports the project, it gets content to its players faster, and it shows the developer that WE CARE. Take your unknowledgable self somewhere else, cause I guarantee you, everyone here will disagree with you.
Oh I know it's hard and lots of personal time and effort goes on. But a demo is there to show off all of that, its a free way to try a tiny bit of a game. I personally don't think asking for money at any point for a demo is wrong.