From ANIMAL PEOPLE, October 1996:

PARADISE, Calif.––Retired
physicist Bob Plumb, now a director of the
Promoting Animal Welfare Society in
Paradise, California, is developing spreadsheet
planning models with help from pet
demographer Karen Johnson of the
National Pet Alliance, into which animal
control and shelter directors can plug whatever
dog and cat population data they have
from their community to project the probable
perimeters of data they don’t have, to
help focus budgets and programs.
For instance, Plumb says, “If we
have 100 cats, half male, a three-year
average lifespan and an average litter size
of four, then each year we need to catch
and kill or adopt out 247 cats to stay at 100.
The yearly cost, at $70 per animal,” the
U.S. norm, “is $17,306. It costs $2,660 to
spay 38 female cats at the start of the year,”
Plumb continues. This is enough to keep
the birth rate equal to normal attrition, “if
we do four more spays each year. The
yearly cost of the spaying needed to maintain
the population, after start-up costs are
paid, is $280.”

Read more