These days, with stricter quality control, factory error miscards are quite rare.
I've seen sellers on eBay pick up on any slight error and sell it for about 10 times the going rate. In the short term these will increase in value, as stock is quite limited and they are selling really well. I'm sure if you put it up for sale at around £100 you would get a bite at that. Long term, prices will settle, but there will always be someone out there who will want this type of figure, just so they have all variants.
Might even be worth paying the extra money and getting it graded, so this will validate that it is a factor sealed card and not one that has been faked.
If your looking to cash in, I would sell it now whilst they are hot. The Stormtrooper figure is going for around £50 at the moment, Chewbacca slightly less. Sell this and buy a new Stormtrooper with the profits is you still need it to complete your collection.