I know it is a tricky question. What do you think, how many Bitcoins are lost forever and are 'out of the network' by lost private keys?
A rough guess of mine will be from a 100-1000 btc from what all stories I have heard about and maybe, maybe there is more that is under the earth for which there is no claim for. A lot of people have started to regret this after the price went up massively and they now don't remember where they kept their bitcoin or what was their login credentials to their account or some other reason. Can people not be more careful? Do this once more then you will regret even worse.
In very beginning people were not giving too much importance to bitcoin, because of the low value of bitcoin, but no doubt that now the may be regretting for the bitcoin which they have forgotten, that where they place their bitcoin. In fact most of the people have forgotten their login to access their wallet. I think through this way a lot of bitcoins are missing permanently.