500K very nice for MN. They will not be much. Even 1M be better.
If you want cheaper MN, go and buy desire, monaco, coinonatx, chaincoin and so on.
Assuming 25% of supply remains in circulation, at 500k number of MNs would not get higher than 100. Won't this challenge the decentralisation? Especially assuming there are a few, like yourselves I'm guessing, with more than a million. Actual true nodes will be estimated at 70 or less.
At least 250k would allow the real number to creep above 100 for a more secure network.
MNs can be max 150 after PoW, but do you forget about PoS? It can be 1000 MNs in a year...