Couldn't relocate the site mentioned in my original reply (it was in Thai but my wife is away for a few days) but have come across another one.
It's your medallion exactly but it's for $1,000! It is however wrongly described as a coin and as Rama VI. Because the seller is located in Bangkok one would have thought that the description would be more precise.http://www.ebay.com/itm/Coin-King-R...em336c8e3bf3
I'm not an expert on these so I really don't know if it is worth that much but if it was for sale in Thailand that's about 30,000 Baht, Whether anyone would pay that for it here I just don't know. I'd just wait and see how that particular auction plays out.