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Not sure how would you deifine "cheap". It's American being cheap now a days when you see how the Chinese torurists here...

To the point, you might want to get China Unicon pre-pay data sim, 3GB or 6GB, 3 months and one year validitiies, reapectively.

Price is about 100 RMB (17USD)per GB in store, half of that online.