an interesting discussion is whether it is more efficient environmentally to use a device (like the lawnmower up thread) that pollutes more than a new one but is already built (and therefore has no marginal cost of construction) versus replacing with new. it is more
than a dollars argument if you are
willing to consider externalized costs, such as the impact on the air, e.g. if your lawnmore pollutes the air, that affects your neighbor (me) and that is a cost which you don't pay but I might (through increased doctor bills or a shorter life). Our "free market" system is not really free, but it's sort of free, and the key defficiency in that is what was first referred to as "the tragedy of the commons". The concept originated in an essay written in 1833 by the British economist
William Forster Lloyd, and it is not getting any less tragic in these days. The basic idea is that if something belongs to everyone, it belongs to no one and it therfore is treated as having no value, with signigicant adverse effects. We have seen this throughout history, it is documented in the Old testament (read about the cedars of Lebanon), it is seen on Easter Island, in the extinction of species and in our problems with pollution and climate change. So, if you take your old tool and you factor in externalized costs, it may or may not be benneficial to change it. If you fire up the old hit and miss engine for 10 minutes every few months to saw a piece of wood, the externalized costs are minimal. If you run it 24X7 powering a generator, not so much.