My startup* actually builds a Products API that provides constantly updated price histories for products sold across the major e-commerce sites (amazon, bestbuy, walmart,etc.. ).

We noticed a three-fold jump in the number of pricing points we collected during the Thanksgiving weekend. Some products actually changed prices in 10-minute intervals.

*Shameless plug: We just launched in closed beta and we have a pretty liberal free plan. Invite link: https://www.semantics3.com/signup?code=ramanujan Drop me a note and I would be most glad to bump up your free quota.