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Worms Clan Wars (hb link) for 1 key + a little more (4 or 5 holiday cards, some gems, something like that)?
Another offer is Worms Clan Wars + Postal 2 for 2 keys.
Terraria, Postal 2, Deponia, Tower Wars <= 1 key each
Shadow Warrior, Trine 2 complete <= 2 key each
[H]
[W]