This exists to some extend.

This would be a website to tell you if the price for a product or service is fair. 

It is simple, a place where people can tell how much they paid and make statistics of it. I am not sure if it would be good if the person told where he adquired that product or service.

The big problem in this is that products or services have many conditions associated with them. Sometimes there are 5 different types of RAM memory capacities, or each color has a price, or in even worse cases, painting an appartment depends on how many square meters does the appartment has. In this case, these details would need to go in. 

A variant of this idea is to do this, collect the prices, and allow the user to make a purchase list. The website would then generate a list that minimizes the cost of the purchase, with the client visiting say 2 or 3 stores. Implied in this is that products need to be specified exaclty.