at level 18, I think it's 53 hp per death he deals and 13 mana, so not nearly as good as people think since he has to spend at least twice that to take out all the minions, but his E provides excelling backup farming and skirmish damage.
by late game, his melee attacks do as much in magic damage as a carry focusing attack speed crits. His Q does do a ton of damage at 100% ap ratio and his W does 65% E 30% R (I think it's also 30% or so but it can be up to 1000 true damage to a champion ) so, he does a ton of damage, but if focused, he goes down pretty quick just because he can't "use" that incredible damage of his. The carry is still your first priority and by the time chogath has attacked you, he still should have like 7 seconds on his cooldowns and will play it safe. If he has used his R, you can go in with little fear of retaliation other than to retreat...if he is out of mana...kill him...kill him good and profit from his likely kill streak