I’ve created simple and free “Price match” module for PrestaShop v1.6 (but it works on PrestaShop ver 1.5 too).

Extension places a price match request button in product detail page and lets customers to send you a request about lower price on the same product in other stores. In administration section you have possibility to process price match requests. You can disable/enable sending letters for customer and/or store administrator about getting new request or processing it.

You can download it by direct link or get it on my GitHub and of course we can discuss this free price match module on PrestaShop forum.