I think the value is relative among the coins in the series. The particular coin you mention the 2007 w proof platinum 1/2 ounce was sold in 4 coin sets, alone, and also in the special set along with the reverse proof. The total mintage for this coin was 25,428 Based on the mintage it is the highest mintage coin of the proof platinum 1/2 ounce coins. At the very worst it will track the price of platinum, at best probably have a small premium over the spot price for many years. If platinum doubles in prices though then the coins value will also double. The lowest mintage 1/2 ounce is the 2008 w proof with 3654.