This would be super amazing if it crawled entire sites.
In fact I had to implement a similar system before any of these nice technologies existed. I was working at a small company in Texas that managed your electricity bill for you in the complicated and deregulated Texas utility market. When I got there, they were manually checking ~100 different utility websites for price changes once a week. When I left, thanks to a system like this, they spent 20 minutes a day manually inputting the price change only on the plans that the system alerted them had changed.