Look for the 50 gal bag. I used the subscribe and save and get them for 8.80 ($10.35 normally) per bag because I order other stuff and get the 15% off. Cheapest I have found them.
The box is still on sale for 44 I recommend getting the box as depending on size of tank will last u a yr fory fifty five I get one box at tax time and it last me till next