They're cheap, but elsewhere they're cheaper: they frequently drop for free on event maps, are always part of the event reward lineups, and then I get several more from the weekly quests.
But the most important point remains: you can't get Whaleship jewelry anywhere else. So far it's been managable to get one of each piece, mostly because I farm more medals in advance when I have the opportunity to do so. If I had prioritized ampies, I would be 3 jewelry pieces behind by now, or would've had to spend about 30 more FGs on stamina refills.