Less than USD 20 and a convincing web page or demo instance: immediately, without even trying it first, if for nothing else then to support a good idea.
Less than USD 50, same rules as above + convince me you are going stick around: once I have some spare cash and after trying your demo.
More than USD 100: you have to convince me.
I'll also be happy to pay reasonable amounts for feature keys etc as they become available.
I'm more hesitant to anything that stops working if I can't pay subscription for a month but in some cases like the old WhatsApp it was a no brainer because of the low cost and low friction.
Edit:
ways to mess it up even if the product is otherwise perfect:
- Windows only or Mac only (I'm primarily a Linux user.)
- Chrome only. (I always was a Firefox guy, but it is now at the point were I actively avoid Chrome)