Dallas is correct in that casting the wand and using the wand are two separate actions. 5 mana to cast it, and however much mana to use the bound spell.
Dissolve + Reverse works like this:
1) Cast Spell step: Enemy player targets your Mage. He intends to dissolve your Storm Drake Hide, so he pays 6 mana during the Cast Spell step.
2) Counter Spell step: You reveal Reverse Magic. The dissolve is countered, and now you control it and use it on your opponent.
NEW SPELL CAST SEQUENCE
1) Cast Spell step: You now target the enemy Mage. (you have no choice, due to the wording of Reverse Magic) Your opponent has already invested 6 mana into the Dissolve. So if you wanted to dissolve a wand (5 mana) you don't need to invest any more Mana. However, let's say you want to dissolve Reflex Boots (9 mana). You will need to pay 3 mana to make up the difference.
2) Counter Spell step: At this point your opponent could use a nullify or his own Reverse, if applicable. But let's say he doesn't have any face down cards.
3) Resolve Spell step: Dissolve resolves. At this point, it has 9 mana invested. You can choose to dissolve any piece of equipment on your opponent so long as its mana cost is 9 or less. Since the only reason you invested mana into it was to dissolve Reflex Boots, you choose to dissolve Reflex Boots.
