Just curious, how much would you pay? Would you feel better about paying per vendor or a flat fee to monitor up to X vendors, or something completely different?

I work on https://monitoro.co which offers exactly what you're looking for, and is not limited to terms of service.

We also allow you to filter changes for the specific ones that are relevant to your needs, and trigger 3rd party APIs or webhooks with the updated data, or a text diff.

I could definitely add in webhook support, feel free to shoot me an email: support@tosnotify.com