Some of the larger Joann shops have rolls of polyester batting in various weights with a price chart posted. You can then feel each weight and see what you like. I believe 4 ounces is the least expensive that they carry. They also have pre-packaged batting in 48" width that regularly sells for $20 - I believe it is 10 yards in length, 48" wide, and 4 ounces or less in weight.
You might like to visit okleequilting.com to see the economy polyester batting that is used for so many Lutheran World Relief quilts. It is 60" wide and 3.5 ounces in weight. I have had good experience with that company, which is located in a small town in Minnesota.
Best wishes on your charity quilting!
Dayle