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GameFly / GameFly UK has GTA titles up to 80% off.
Grand Theft Auto III $2.49 (Steam)
Grand Theft Auto IV $5 (Steam)
Grand Theft Auto IV: Complete Edition $6 (Steam)
Grand Theft Auto: Episodes from Liberty City $5 (Steam)
Grand Theft Auto: San Andreas $3.75 (Steam)
Grand Theft Auto: Vice City $2.49 (Steam)