So, how can the difficulty be zero?
Because when you account for all of the above, the block reward is gained at effectively zero cost.
First of all, the miner takes a higher risk than the masternode holder because if they invest in $77,000 USD of mining equipment it will start depreciating immediately and lose most of its capital value over a couple of years. There's no upside there and it's the depreciation that counts as a cost against the mining reward, not the capital cost. So unless you plan for a constantly devaluing Dash, this is zero in the case of the masternode and near 100% in the case of the mining equipment.
Secondly, the masternode operating costs you cite of $5-$20 Per month against a $390 reward yields a $370 profit even at your upper range hosting cost (=95% profitability). None of that profit stays in the network.
Meanwhile mining incurs the cost of scarcity by paying for the coin they mine according to the prevailing difficulty level. In doing so it contributes further to that scarcity by raising the difficulty level.
Currency/opportunity risk are not accountable operating costs and are common to any economic activity.
We've heard some FUD over the past several pages, let's examine the situation.
the SPORK on the 17th of Nov to fork off about a 1000 legacy nodes clinging to old versions. 
Once the spork is active, the network will become one again and joyous time can he had by all. I do expect to see some selling from bears that would rather the network didn't upgrade and to that end I have put together a fantastic resource for us!
We also provide the current PoSe score and address balance. Once the SPORK is flipped, the network will start PoSe banning these nodes and we can expect them to act. Either they upgrade, or they sell. This page is how you can track that.
