> I'd be willing to pay between $10 and $20 USD
I'd argue that seems grossly optimistic for most users. Considering that many useful tools are free and most people balk at paying $6.99 for an app.
How about a $0 starter tier for max. of x monitors, with a max freq. of every y hrs... then move up to a one-time fee that is more generous, but still constrained, and finally have a monthly fee that allows unlimited.