Every event shop and what the event really pays back. Each shop page ranks its items by gem value; the events tagged ROI also add up what you spent in real money and what you received, day by day.
Time-limited offers: dedicated currency, capped stock and an end date.
Five events are broken down day by day: Brewmaster Stall, Moonlight Shop, Theater Shop, Dragon's Caravan and Adventure Stall. Their page adds a section under the item table where you tick the packs you bought, and it totals what came back.
The percentage is the value received divided by the money spent, both in the same currency. Nothing converts gems into euros: the gem value sits beside the percentage, never inside it. Spend nothing and there is no percentage to show - the tile says F2P.
One caveat worth knowing: an item is only worth what you would have bought anyway. A pack full of speedups you will never burn scores well and still is not a good purchase.
Elsewhere in Value & ROI