If the balance is less than ¥180, it doesn't worth the $20.

You can get such a data SIM card anywhere in e-market (like Zhongguancun in Beijing) with a discount of 20% to 30%. A ¥600-worth SIM card may sell for only ¥480. Once you used up this balance, just throw it away and buy a new card at discount rate. I always do it like this.

Official rate (monthly):
• 1 GB – ¥80
• 3 GB – ¥150
• 5 GB – can't remember, maybe ¥200 or ¥250?
• 10 GB - ¥300
• maximum 15 GB - ¥800