0.00068164/0.0675 * 52 = 52%
You invest 1btc and two years later you have 1 btc plus the value of your share.

The value of share depends if we can overpass and grow faster than the difficulty.
If we stay like this (comparatively to diff) I think the real value is somewhere near 0.1/share when people will have some trust.
If the bitcoin reach again $1000 or why not $2000 and is popular in shops and consumers wallets, the share value would be... may be... 1 BTC