Comparative stats for the Skeleton King are circulating:
Skeleton King
Normal: Lv.8, 1800HP
Nightmare: Lv.33, 123,250HP
Hell: Lv.52, 2,471,100HP
Inferno: Lv.61, 15,717,300HP
Looks like the official D3 game guide has a sample page on the iStore, be wary of spoilers.
Edit - Normal Monsters :S
Armaddon (appears in act IV)
Normal: lv.29, 1200-1700hp damage per hit: 443
Nightmare: lv.49, 12k-17k hp damage per hit: 3445
Hell: lv.60, 54k-76k HP damage per hit: 14k
Inferno: lv.63, 306-428k HP damage per hit: 170k (!)
Grain of salt as always.
Very interesting, doing quick calculation on how next level relates to previous:
Moving from:
normal ---->> nightmare = 68x more hp
nightmare ----->> hell = 20x more hp
hell ----->> inferno 6x more hp
Now comparing average weapon damage increase.
I will take average damage (min + max / 2) for 2 handed sword for levels that you would normally fight skele king ( or about that at least from my personal guess or whatever closest level weapon I will find to what I think it will be ).
Normal = Level 9 sword (1.1atk speed) - 12-17dmgdps = 14.5dmg average
Nightmare = Level 32 sword (1.1atk speed) - 52.8–54.5dps = 53.5dmg average
Hell = Level 51 sword ( 1.1 atk speed) - 171.6–179.9dps = 176dmg average
Inferno = Level 60 sword ( 1.1 atk speed) - 367.4–379.0 = 373dmg average
Now moves ( rounded up to closest 0.5x)
Normal ---->> Nightmare = 4x more damage
Nightmare ---->> Hell = 3x more damage
Hell ---->> Inferno = 2x more damage
Looks like move from normal to nightmare will be massive and then it will be more linear and slow increases.
You can already guess that beating the first pack of zombies on nightmare will take you around 15x longer than on normal comparing weapon damage increase to hp increase.