What is initial_key in genesis.json and role of this property?
initial_key is public key of genesis nodeos, this property used on genesis nodeos to start producing. All other nodes those who want to participate in the network should have the same key in their genesis.json file.
How is the initial key generated?
$cleos create key
I have just checked that there are around 400.000 ETH accounts and on my local node a new account takes 2.66 Kb memory.
Is it a safe assuption that the starting memory will be 64 Gb = 64*1024*1024*1024 and the occupied RAM will be 400.000 * 2,66 * 1024, so at launch around 1,6% RAM will be used, so the cost of account creation at launch will be around 0,016 ...