I used browser extension Distill in the past, it’s pretty easy to use and it works well for detecting/tracking changes of the specific elements on the page. I think free version allows 25 local monitors.

I also just found this extension Automa, I’ve never used it, but it seems cool. Looks like a Tasker for your browser. And there are also a workflows that people share, I saw this one randomly, Scrap Google Suggest to SpreadSheet so I guess you could do a similar thing for prices.