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Cities XL 2012 is on sale for $9.99 (75% off) at GMG as a daily deal. The special price expires 1100 UTC 9/19.
This does not activate on Steam. It uses the GMG capsule program.
BTW, you should be able use to the current 20% off voucher code to make it $8. Click here to see the current promo codes, coupons, and special offers page.