Instant? Maybe every hour?
Yes, such things do exist. But hey do not search the
whole web. Instead, they search a number of vendors that compete in your area of interest.
It depends on what your are seeking.
Here is a place to start your quest:
https://www.shopify.com/blog/7068398-10-best-comparison-shopping-engines-to-increase-ecommerce-salesComparison shopping engines collect product information, including pricing, from participating retailers and then display that collective information on a single results page in response to a shopper's search query. In this way, shoppers can compare each retailer's price, shipping options, and service on a single page and choose the merchant that offers the best overall value.
After that, take a longer look at:
http://www.pricegrabber.com/and:
http://www.pricewatch.com/After you play with them, notice that they have some kind of way to send emails whenever a change has been found. You ought to have a separate e-mail for this.
Does that help?