I joined in on a game around level 30k, and when autobuy ran out of money trying to keep me alive, it started buying the lower level armors until they were more expensive than the higher level ones. It doesn't seem to factor in the gain per cost like the script does for damage upgrades. I currently have 20 light armor, 18 heavy armor, 15 energy shields, 12 personal training, and 9 cybernetic enhancements, and I haven't bought anything manually. This makes all of them cost around 12-15 million each, even the ones that give negligible health.