That’s an absurd cost for problem with such a simple solution. I bit the bullet with an app called Jump, which has as many features as I’d ever want for this. I wasn’t happy putting up $8 for it, in principle, but at least it isn’t a choice between an ad or a subscription.
I mean they’re almost certainly targeting businesses for whom $20 for a lifetime license might as well be free.
Charging that much is also a way to reduce support costs since anyone who will fork over that much for a VNC viewer probably knows what they’re doing or has an IT department.
https://edovia.com/en/screens-ios/