I can confirm that on both Hard AND Suicidal, my earlier statement is correct.
.5 Hand Grenades per box
.25 Pipe Bombs per box
If you die, the counter seems to start over, as well. If you were at .75 of a pipe bomb and bite the big one, when you respawn, you're at .0 of a pipebomb again.
Not sure if reloading your pipes has any effect on this, though. Boxes are so rare on Hard/Suicidal, however, that getting pipes from them isn't viable and getting nades from them is still pretty wasteful. Don't take ammo boxes unless it does more than give you grenades...seriously!
On Beginner/Normal, the boxes give more ammo, so I'm not sure about those. I stopped playing Normal a long time ago.