The HMMWV was never intended to carry armor. Stop trying to make it something it isn't. Instead, look to inexpensive armored cars as an alternative, such as the South African Ratel 20 or similar (or the Alabama Slammer).
The HMMWV was always intended as a heavier (and more stable) Jeep.
The HMMWV was designed during the height of the cold war, as a fast personnel mover capable of traversing the European battlefield. The assumption was that it would function in an armor-battle environment, where anything likely to hit it would erase it completely, armored or not (hence the designers didn't worry much about it). It was never intended to function as a convoy protector, or as an up-armored patrol vehicle (that's what the bradley was supposed to be for). I expect that somewhere in the next generation of military vehicles, there will be designs for the kind of environment we are seeing now (chaotic, no central points of resistance that can be easily avoided, more diverse terrain).
Lots of nice pics and info on performance/upgrades here. http://www.fas.org/man/dod-101/sys/land/m113.htm