Unitil the event is over it will be difficult to say, even at that point the total number spawned will likely be impossible to calculate, and its very possible EA will relocate the invasion artifacts to a dungeon or other location.
To answer your question though I have not seen any for sale but I have observed individuals interested in purchasing one, offering about 10-15m.
Mind you if no one is willing to sell this item then the value will be ZERO...