Why we have to compare the investment by BTC?
Think about if you invest gold or silver, how you judge win or lose?
I bought gold and silver with the intention of one day spending it as gold or silver coins for goods, not for the intention to trade them for FRN. But yes, you have a point there. Ideally I would like to invest some of my silver coins into something that pays me more silver coins in the end instead of hoarding them.
Compare with GPU's power consumption, I don't know to judge Avalon buyers win or lose?
One Avalon (3 modules) is costing you 600 Watts thus 14.4 kWh / day. Say at EUR 0.22 / kWh, it's EUR 3 / day
Every day the Avalon (at current difficulty) is generating about BTC 4.
You need to sell each day BTC 0.02 to cover your costs. The rest pays of your investment until you break even
GPU systems consume far more Watt / GH/s
Therefore, I think the topic is totally wrong. When I saw this "I need 20 day to get invest back.". I have a feeling which in Chinese is "得了便宜还卖乖！" I don't know how to translate to English.
I think 20 days is optimistic 60 days is more realistic with increasing difficulty. Maybe 90 days for batch #3