I think a more investor-oriented way to look at it is per amount of holding. The reason the 'incentivisation' works is because of a gain on holdings. I don't think people care how many masternodes they're personally running. They measure it as a return on holdings, so if you've got 1k Dash you want to invest that whole amount to get a return. 1 masternode is 1 unit of expense to get a fixed return on that 1000 Dash. 2 masternodes is 2 units of expense, so the expenses double for the same return.

For that reason I don't think you'd see a doubling of masternodes with a halving of the collateral.  (At least not automatically. If the dollar price doubled then possibly).