、。
It wasn't a hundred-yen coin — it was a bottle cap.
Literal
Hundred-yen not-being, crown-cap was.
百円じゃなくて ('it wasn't 100 yen') uses the casual negative copula. 王冠 (おうかん) literally means 'crown' but here refers to a bottle cap (crown cap) — a common meaning that catches learners off guard. でした is the polite past copula. The sentence has a punchline structure — the expectation (a coin) is corrected to something worthless (a bottle cap).