The materials and metals that are finite also put a “natural” limit on the increase of biological organisms. But, I do understand what you are saying. Upkeep on humans is remarkably low, what with the self healing an all. The real problem is when self healing machines are created, or nano tech. The costs to create and upkeep machines will decrease, the Costs of humans will decrease also… But, impart because of the decrease in costs of machines.
I forgot to include emotional costs for humans, loosing a human has a greater emotional cost than that of a machine, it’s just stuff after all. Also, the intelligence* that a human has that far exceeds (at this point) robots adds to the value of humans.
Though, the underline argument I made is actual real world application. We use machines in situations we could not use a human. The moment a type writer was used instead of something being written by human hands, machines replaced humans. The moment that a plow machine was used instead of a scythe, machines replaced humans. The Luddites smashed machines for a reason… The reason why is because machines are cheaper than humans and human life, they can be thrown away at a lesser cost than a human. Anytime you buy a new toaster it is because that machine is cheaper than starting a fire and cooking it yourself. More, the replacement of humans for machines has increased (at least to most value systems) the life style of most people. Trust me, you do not want humans to be cheaper than a machine, the results are less enjoyable.
An example: In India, where dowries are acceptable, a newspaper ad once read: Man interested in marriage for tractor. Send pictures of tractor.
If human life is cheaper, than people are thrown away before the machine is, I know from other posts that you do not want this. Children are the cheapest of humans, because they have not had as much energy placed in them.
When sex slavery happens it is because the cost of that human is lower than the costs to, well, a lot of things.
The act of creating the immorality of children being used as labor increases the costs of human life as a whole.
I should probably point out, I was answering a specific question:
I have not commented on the OP, nor should my arguments for machines being cheaper be taken as agreement or argument against the OP.