I was unable to cast my opponent's Force of Negation by exiling a blue card from my hand after I got it with Siphon Insight. This is clearly a bug, as on Siphon Insight's own rulings it even mentions how to determine alternative costs: "To determine the total cost of a spell, start with the mana cost or alternative cost (such as a flashback cost) you're paying, add any cost increases, then apply any cost reductions. The mana value of the spell is determined only by its mana cost, no matter what the total cost to cast the spell was."