We also both forgot to throw in Elixer of ruin for an extra 250hp

In your example case, where you used Cho'Gath as the most OP example of this item, however I would like to mention that your build would be ****. You would have 0% CDR. Tanks excel with CDR more than most any other champs. What makes them strong is the cc in their abilities and the ability to use those abilities a lot. A tank with no damage, no CDR, but tanky as **** is pretty much a non-factor in a team fight. To be relevant, you either have to build damage so you can be a threat or CDR so that you can be a CC machine that either helps peel or dive. If you can only do one rotation of spells in a fight (because they are on CD), you might be able to not die, but you aren't helping your team either.

Using cho'gath as an example, a more realistic build would be Cinderhulk->Frozen Heart->Merc Treads->Randuins->Locket->Warmogs/Banshees/Something Else . This build would give you 30% CDR and would allow you to take 5% from runes and 5% from masteries, letting you hit the 40% CDR cap.

Now to find the bonus health from this build, we're going to go ahead and use Warmogs for the most possible health.

900 from ult
350 from CinderHulk
500 from Omen
400 from Locket
800 from Warmogs

2950 Bonus Health from Items
737.5 Health from Cinderhulk Passive

1934 + 2950 + 737.5 = 5621.5 Total Health (Before runes/masteries)

The total health you are getting from Cinderhulk is 1087.5

Cheapest Health item is Ruby Crystal 150 Health for 400 Gold 2.667 Gold/health

The effective value of Cinderhulk's passive late game is worth 2900. Keep in mind, this is on a champion who can get the absolute most bonus health while still being useful in terms of CC.