Does Isochron Scepter let you cast a spell imprinted on it for free or do you still pay the cost?

Isochron Scepter's activated ability (costing {2} to activate) lets you create a copy of the imprinted instant card and, if you do, you may cast that copy without paying its mana cost. So the {2} activation cost is the only mana you spend — the spell itself is free.

Under CR 601.2b and the concept of 'without paying its mana cost' (CR 601.2b, 117.6), when an effect says you may cast a spell without paying its mana cost, you simply skip paying the mana cost portion entirely. You still follow all other rules for casting the spell, including choosing targets and making other decisions.

Importantly, even though the spell is free, you can still pay additional costs (like kicker) or have mandatory additional costs apply. You just don't pay the printed mana cost (CR 601.2b, 117.8a).

Concretely: suppose you imprint Lightning Bolt (normally {R}) on Isochron Scepter. You pay {2} to activate the Scepter, make a copy of Lightning Bolt, and cast that copy dealing 3 damage to any target — without spending any red mana at all.

One nuance: you are casting a copy of the card, not the card itself, so the original imprinted card remains on the Scepter for future use (CR 707.12).