Actually it was Bigelf and Scale the Summit that replaced Pain of Salvation and Beardfish on the North American leg of Prog Nation 2009 - the band Three was the opening band on the previous Prog Nation tour in 2008.
As for how rare it is, I don't know for certain, but it is probably somewhat rare given the fact that PoS and BF ended up not being on the tour. All posters sold during the tour would have had Bigelf and StS listed. In terms of where to offer it (I assume you mean to sell it), eBay is always there. However, you might consider announcing that it is for sale on different message boards like this one, and see if any one is interested. Regarding pricing, I haven't a clue - probably best to list it as "best offer" and take it from there. If no one offers you a price that you think is reasonable, you can try selling it on eBay or just continue to hang on to it.