I would say $3-$5 for a common in p/f. $5-$10 in f/g $10-$15 in vg+, then prices jump as they get nicer and are graded. There are plenty for sale, so you can bargain hunt and take your time. I sound like a broken record, but I look at eBay past sales for a general idea on pricing.
www.ebay.com/itm/164081239596