everything below 10$ a piece have to be considered as failure
I think to be on the safe side people should be aiming long-term to get to 1% of total coins. At that amount one can easily live off interest rate, assuming the price reaches 10$ level. People then effectively become banks that issue coins and because it's done though PoS there is no need to instantly sell them to recover electricity cost.
Investing in Diamond Cloud Mining is definitely something everyone who cares about their investment should do. First of all it supports DMD's price and with enough constant feed the price would eventually go up. Secondly, you get constant payouts which bundles nicely with in wallet PoS staking. So you sell minted coins but at the same time you are constantly topped up with new coins. One can set everything up in such way that even though u sell your stash you gain coins at the same time. Win-win situation.
As to the price of DMD.. as cryptonit put it 10$ is good base price , and we base many of our predictions on this moderate price rise.