I wrote up a comparison of scuba gear merchants based on price: The hunt for dive gear bargains. The merchant score was calculated by looking at products that are offered by multiple stores and calculating the average difference to the lowest price. With this method, zero is the highest score — it means that a merchant's prices are always lower than the competition.
It'd be great to have the community's feedback on this approach. Do you think there is a better way? One obvious improvement is including shipping costs, but that's left for future iterations as it is pretty tricky.