So your basic formula at max diff and min reward of 5 drk/block, you'd be looking at 576 blocks * 5 drk/block = 2880 drk daily * 10% masternode reward = 288 drk / 686 mn = 0.419825 drk/node/day. At the current price of $6.75, you're looking at $2.83/day per masternode or around $85/month. Again, a lot of assumptions here. Still not bad considering you can rent a server for less than $8-$10/month.
There will probably be more, as for example my DRK is split in 4 separate addresses, none of them more than 1000. But I could put them together to create couple of master nodes, if I want to.
I think probably less, not everyone holding 1k+ is going to setup a MN.
Indeed.
Re the calculation, if all these addresses decided to set up a masternode that make 686000 DRK taken out of circulation which makes less coins available to buy, which in turn will up the price. I was making some provisional calculations and came to a conclusion that at 1000 MN there we would be looking at around $10-12 per DRK and around 0.288 DRK a day for masternode operators.