Isomergames via Amazon offers Prime members Catherine for PlayStation 3 (pictured) for $17.49 with free shipping. (Non-members can spend $25 or more to qualify for free shipping; otherwise, shipping costs $4.98.) That's $3 under our January mention and the lowest total price we've seen for this game on any platform. (It's a current price low by $6.) Sales tax is added where applicable.
Isomergames via Amazon also offers Prime members Catherine for Xbox 360 for $17.59 with free shipping. That's $2 under our January mention and the lowest total price we could find by $5.