Cost for existing: A standard, constant cost every single bot has to pay just to exist.
The existing aging cost supersets this. It is a cost for mearly existing that you can choose to begin assessing at a specific age (including from birth) and can choose to increase logrithmicly over time if you want.
Body Cost: Loss of Body (think like how slime dissipates). It'll be reduced by shell or slime (though shell will grant much more resistance then slime, while slime will just more or less take it's place.)
All costs are paid in nrg today. While slime, etc. will dissipate, this is not a cost, it's just an artifact of the way slime works. The Body Upkeep cost is a cost on how much body a bot has. The more it has, the more it pays, but like all costs, it pays it in nrg. If it does not contain a gene for converting body to nrg, it will die young and fat.
Keep the suggestions coming, but unless there is a good reason, I am not inclined to add a new class of costs that take from body instead of from nrg. If you feel strongly, about this, I'd like to better understand the requirement and why the Body Upkeep cost is insufficient.