I worked for a company that did exactly this many years ago. (They were even able to parter with some retailer). Their product worked well yet they still went out of business long ago.
To be honest, I don't see much value in such a service, not that it doesn't exist, it's just hard to justify paying for this data.
I’m asking this because I run a small side project to show prices across retailers for a very small niche. The users are very very happy. Even the vendors started contacting to be listed on the comparison.
But I am unable to make a business out of it other than few affiliate commission.