One recently sold on
ebay for $1.34 (click on the below link). There are hopeful sellers on
ebay who start auctions with totally silly prices for such things, but of course they always wind up with zero bids. They are fishing for unknowledgeable bidders hoping to catch someone. The thing about this particular stamp is that they printed more than 152 million of them making it one of the most common commemorative stamps ever issued. There was extensive interest in creating all kinds of first day covers and items with first day cancellations so these things are really common. Such things commonly sell for around $1 each. While you should be able to multiply that $1 times the number you have and then add a few dollars more for having a set, your problem is that you do not have a complete set. There were 12 different ones in this set but you mention that you only have 11 of them.
http://www.ebay.com/itm/First-Day-I...047675.l2557