Companion note: the first on-page poster is Ch. 96 (Marine HQ wanted-poster wall, Brandnew’s briefing); Ch. 98 is the first Marine-officer-side voice-narration of the bounty number.
Why this matters
First canonical Luffy bounty — the anchor for all forward bounty-comparison math in the early arcs.
Bounty-rarity register installed: 30M is a Loguetown-Marine-officer’s once-in-a-while case — the reader is being told that East Blue 30M is significant by regional standards, but the Grand Line is forward-pre-loaded as much higher.
Bounty-as-defeat-response grammar canonized: Luffy defeats a 20M Arlong → Luffy bounty exceeds 20M (set at 30M). Every later Luffy-bounty (60M post-Alabasta-Crocodile, 100M post-Skypiea / Enies Lobby etc.) is a scaled response.
First-Marine-recognition-of-the-Straw-Hat-captain moment: the ‘Luffy!? Never heard of him.’ line installs the new-pirate-emerges-and-becomes-known register.
First Grand-Line on-page acknowledgement of the bounty