Ruling: Summon History

 
The cost for Summon History has to be declared as the card is played (before searching). If you then retrieve a card that costs less than the declared cost, the full cost is still paid. Obviously, you cannot search for a card that costs more than the declared cost.

Reference:
http://groups.google.com/group/rec.games.trading-cards.jyhad/browse_thread/thread/31e052470dce242d#