What is the price on society for having too many pets? This is hard to judge, and naturally will be different worldwide, especially as every nation has a different method of handling pet overpopulation.
Some nations seemingly avoid a financial burden by allowing unwanted pets to roam the streets. It must be noted that in these countries the people find methods of dealing with the problem on their own. Often throwing dogs into old well shafts or off bridges to die. If not, these feral animals form into packs and become a danger to people, livestock, and cared for pets. The cost is apathy.
Other nations see the surplus animals as a food supply. Why not? Nobody wants to deal with them and their numbers become a plight to society. The cost is hard to measure but there may be health concerns with eating animals whose diet mainly consists of meat.
The biggest costs associated with pet overpopulation are felt in developed nations where the burden of excess animals falls on government hands or into the laps of charities. City pounds, animal shelters, veterinarians, and pet rescues, all deal with surplus animals on a daily basis.
Most areas have some form of animal control, by-laws stating how many pets a person can own. Many have officers in charge of picking up reported strays or impounding animals when people are found with more than they are allowed. These people are generally not volunteers, but are often trained, professionals, paid for by the city or county.
Once the animal is picked up, it must be housed. This falls onto the city pound or shelter. A building must be used, and staff employed to care for the animals. Typically people have a minimum of three days to pick up their impounded animals, if nobody does claim them they become property of the state, or shelter, and become its financial responsibility and burden. Some animals arrive at the shelter as unwanted pets, excess litters, or for other reasons. We know that far more pets are born every year than there are homes for and most of these animals then become a financial liability.
Although statistics vary, fewer than half the number of cats admitted to shelters every year find homes, while more than half of the number of dogs do. Even no-kill shelters turn pets away when they are full.
Most shelters have paid staff and volunteers to help care for the animals and raise funds. Some shelters receive government funding, but not all do, and generally it is not enough to cover the full budget. The cost of pet overpopulation means many human hours are spent fund raising.
There is no way every unwanted animal can be kept alive. Shelters would soon be so overcrowded and overburdened they could not cope. This is why so many are forced to euthanize excess animals. Euthanasia is not free, shelters must pay veterinarians for this service (although many shelters get discounted rates).
The cost to veterinarians who routinely are called upon to euthanize an animal for no other reason that it being born is an emotion one to a person who has entered a profession because of a desire to help animals. Their time is taken up by the need to remove some unwanted pets from the system.
Finally there is the emotional toll taken on by the staff who come to love the animals, only to see them wait for homes that do not exist. In an irony sometimes they are accused of not caring, and some members of the public refuse to donate to shelters that do euthanize pets, thus making an already bad situation even worse, and punishing both the animals and people trying to help them. Everyone involved in animal rescue pays the dearest price by witnessing something they know is 100% preventable if only people would spay or neuter their pets, and not allow intact animals to roam. As such the biggest cost of having an overpopulation of pets is what it does to people who want to help.