Provide shelter, food, attention, medical care etc to abandoned (or other)
domesticated animals. Project will scale from providing sanctuary for an
individual animal in your own home, to creating a massive home for hundreds.
Depends on scale. At the smaller scale, the only significant cost is likely
to be in medical care and perhaps time (dogs need to be exercised etc).
No clear upper limit on scale or cost.
The value of life over death for the animals concerned. The animals should
be better-off than wild animals, due to human and medical care. However the
cost is likely be much higher than in increasing wild populations.