There's no such thing as an 'ideal' amount for someone's first deposit, it doesn't exist as it's completely subjective.
I'd like to reference Ivey's often-misunderstood quote:- 'If you can't take $100 from your wallet right now and burn it, you shouldn't be playing poker'...
My advice to any new players, especially inexperienced ones, would be to build their bankroll from freerolls.
This will allow them to gain experience without needing to outlay money and simultaneously get somewhat of a 'feel' for how the game plays online.
As for more experienced players, their buy in should probably be based on how much they can comfortably afford to put aside in bankroll, the stakes they intend to play and any deposit bonuses available.
Anyone interested in this subject should research 'BRM'- bankroll management.