Holiday promo: Mishra's Toy Workshop

Every year on the occasion of Christmas time Wizards print a special card given only to its employees and sanctioned games stores. This year's holiday promo is Mishra's Toy Workshop. How could we obtain one? Try eBay, it goes for circa 100 USD. However, if you are thinking right now whether it is a good investment I will dispel your doubts very soon. Assuming, the number of promotional cards released every year is approximately constant and knowing the fact that only collectors purchase such cards, due to its illegality in sanctioned tournaments we may conclude that each year a newly released Chrismas promo card in December was worth 100 USD let's analyse the price of these promo cards using eBay as a tool to quotation.
2013: Stocking Tiger, 45 USD
2012: Naughty / Nice, 40 USD
2011: Yule Ooze, 40 USD
2010: Snow Mercy, 68 USD
2009: Season's Beatings, 54 USD
2008: Evil Presents, 55 USD
2007: Gifts Given, 98 USD
2006: Fruitcake Elemental, 49 USD

As the times goes on investors expect the higher return rate on investment. However, the data presented above shows that regardless how many years passed the price settles at give or take 50 USD. A strategy to buy and hold a card for few years and observe how the profit is steadily raising doesn't work here. So, putting it all together - holiday promo cards are terrible investment, unless you may get them really cheap.