How can I determine the value of very rare cards?
The only way to attempt to determine a concrete value for a card is to find it's price in a large market place where market forces (supply &amp; demand) have determined the value of the listed cards.

Europe's biggest trading site, magiccardmarket.eu, is very comprehensive in the cards that it lists. They have one of the best listings of uncommon sets.
There the winter orb from the World Championship Deck's in near mint condition is ~2€

As my predecessor already mentioned, it is not tournament legal (only white and black bordered cards are).

Nevertheless, gold border cards are a cheap alternative for players building a draft-cube or for any other non-tournament use.