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BioShock 1 & 2 for the Mac are on sale for 50% off at GamersGate / GamersGate UK.
BioShock (Mac) = $9.98 / £6.98
BioShock 2 (Mac) = $12.50 / £8.75
Both titles are actually slightly cheaper at the MacGameStore (USD at least). BioShock 1 is on sale for $9.95 and BioShock 2 is on sale for $11.95.