The cheapest (per GH/s) mining gear that is available right now are ASICMiner's Block Erupter blades. You can get 10.7 GH/s for
BTC 3.65 including shipping, and even less if you oiroder more than one blade.
At the current difficulty, 10.7 GH/s should earn around
BTC 0.0477 every day. Assuming the difficulty rises 20 % in every adjustment, you'll have 11.67 days between adjustments. If you started mining when the difficulty jumped to its current value, you would have made
BTC 0.5566 before the first adjustment (minus pool fees and power costs).
Sticking with 20 % difficulty increase every 2016 blocks and neglecting the fact that block rewards will eventually get lower, you won't mine more than
BTC 3.3396 (see
geometric series), which is
BTC 0.3104 less than your investment...