[llamabeast]

I think the above answers are still somehow ambiguous so I will try to give an unambiguous answer.

Let's say your opponent has cast Arcane Nexus, which costs 150 gems if I remember rightly, and he used a total of 180 gems, so 30 extra.

To dispel it, you need 30 for the dispel + 30 extra (plus a few more to be on the safe side), so a little over 60 gems. So yes, in (almost?) all cases dispelling is cheaper than casting.