I just got a Cup of Sugar from the Egg & Dairy trade in the Royal Exchange - would've been better if it was Candied Sprinkles but still...
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Anyway, I've compiled a list of the known drops so far for the different Trades. Maybe it'll help others decide where to put their efforts. I'll add to the list if I come across any other drops.
Bronze Prize Trade drops 1 item
- Sweet Cream x1
- Sugar Crystals x1
- Cup of Sugar x1
- Candied Sprinkles x1
- Ice Cream (for event) x1
- Sugar x3
- Sugar Cane x8
Silver Prize Trade:
drops 2 items
- Sugar Crystal x1
- Sweet Cream x2
- Candied Sprinkles x1
- Royal Sundae (for event) x1
- Ice Cream (non-event) x1
- Chocolate x2
- Syrup x2
- Mithril x1
- Fancy Block x1
- Fancy Beam x1
- Baby White Sheep x1
??? Prize Trade: drops 1-2 items (usually 1 item, rarely 2)
- Gold Nugget x2
- Gem x1
- Energy x3
- Expansion Permit x1
- Sundae Shrub (there are a few different colors)
- Lv.1 Ice Cream Cottage (upgrades same as standard Cottage)
- Lv.1 Ice Cream Spire (upgrades same as standard Spire / counts toward Royal Building total)
- Lv.1 Ice Cream Watchtower
- Sprinklepea
- Alicorn