Sanctuary - The Complete Fourth Season [Blu-ray]: Amanda Tapping, Robin Dunne, Christopher… for $15.99
Sanctuary season 4 on Blu-ray is currently at 15.99, the the lowest price I've seen thus far (and lower than the previous Blu-ray deal of the week price). Eligible for Amazon Prime, or free super-saver shipping with $25 order.
Other seasons are also available, although season 3 is still somewhat higher at the moment for whatever reason:
Season 1, 16.49: http://www.amazon.com/Sanctuary-Complete-First-Season-Blu-ray/dp/B003BV8I6I
Season 2, 14.99: http://www.amazon.com/Sanctuary-Complete-Second-Season-Blu-ray/dp/B003BV8I6S
Season 3: 20.49: http://www.amazon.com/Sanctuary-Complete-Third-Season-Blu-ray/dp/B0057FGCZK
Season 4: http://www.amazon.com/Sanctuary-Complete-Fourth-Season-Blu-ray/dp/B007XF0XLQ