I think that is really a hard question. Around here I have seen them from $25-$30 each. Yet I do no they go for less other places and higher other places. If you are planning on buying you need to check your local antique shops. I was looking online and found this http://stores.ebay.com/Jeannies-Feedsacks-and-Textiles. I think the prices are pretty good, maybe you should check them out.
Jeanna Farmgirl Sister #41
Go confidently in the direction of your dreams. Live the life you have imagined. Henry David Thoreau
Here is the link,you needed to leave the period off the end Depending on the rarity of the feed sack, age, condition etc that will effect the prices. I have seen them range from $9.00 up to $45.00 so it really will be up to you what you would like to pay for the piece. Happy hunting!