It's more like 30+ % increase in difficulty per month minimum... it's just that the mining reward is decreased at 4-year intervals.
Don't make hardware decisions until you understand the dynamics at work here, in case of doubt, go to: http://www.bitcoinx.com/profit/

• raise difficulty by 45-60% to account for 4-5 weeks shipping
• reduce conversion rate by 25%
• reduce hash rate by 10%
• increase elecricity rate by 20%
• increase power consumption by 20%
• increase pofitability decline by 15%

=> check if you're still breaking even under these cirumstances (and WHEN) - adjust your numbers accordingly

PS: You are doing something wrong if you're buying erupters, you should be SELLING them !