(bold text = witnessed, italic text = assumption)
In general: Girl level determines the chance to get relics, never the amount.
Well, previous idea of 80-point intervals seems off since a donut on Premium only gave 2 relics, the same as a free bath item on Premium. And I've got video evidence of that donut; what that makes me question things now is the Pheromone Spray, as that has the same restoration value yet awarded as much as a donut on Premium, and that data is from before I started recording everything so I can't reconfirm it. Could be wrong, could be a difference between individual items, could be girl preferences (P Spray was with Cleo, who likes it, while Donut was with Valentina, who dislikes donuts), can't say for sure.
Actually, wait. I just looked in the shop, and the Pheromone Spray is techincally a higher rarity item than the Donut, despite being the exact same value (Green/Common < Blue/Rare < Purple/Epic < Gold/Legendary, (rarity names based on clothing) P Spray is Blue/Rare while Donut is Green/Common). That might be the deciding factor in payout and not the actual point values. Will update the above entries to reflect the consumable rarities where I can. The clincher for this theory would be Sturgeon Caviar awarding relics on par with Yubari Melons / Racon / Gold Glitter, or at least more than the rare tier (so more than the 120-200 non-gem consumables), since it's an epic-tier item with only common-tier stat restoration.
Garden items are a whole 'nother matter since, as far as I can tell, there's no way to check their rarity. However, I'd assume that Tier 1 crops are Common, Tier 2 are Rare, and Tier 3 are Epic for now, as the Pumpkins and Lilys seem to provide large relic amounts (though current numbers are only with Premium active). Unsure if quality actually matters now that I'm working on the assumption that item rarity, not points restored, is the key factor, but data on low-star high-tier crops could be useful for this. -Cobalt