Is Berlin cheaper than Amsterdam?

Generally, yes: Berlin is usually cheaper than Amsterdam, especially for accommodation and many day-to-day costs like eating out and nightlife. Amsterdam is often more expensive overall, particularly for housing and lodging.

That said, the gap depends a lot on the neighborhood, season, and what kind of trip or housing situation you mean. Berlin is no longer especially cheap in the most central areas, and Amsterdam can sometimes be less expensive than expected if you book early or stay outside the most tourist-heavy areas.