Today in the USA, total income for private animal protection organizations is around $2.44 billion or just under $8 per capita. However, there are large variations in this figure from one state to another ranging from Colorado at the upper end (around $13 per capita (DC is actually higher at $16‐17 per capita but a lot of money comes in from the outer suburbs in Maryland and Virginia) to a low of around $2 per capita in Utah (note: national groups like Best Friends are not included in the income numbers for each state). If every state were as successful as Colorado, then the animal movement would be bringing in $4 billion a year (almost double its current income). In fact, there are signs that the animal movement could have the potential to raise even more than this. San Francisco, for example, raises over $30 per capita on dog and cat sheltering, rescue and sterilization and Marin Humane Society (across SF Bay) spends over $20 per capita. At the other extreme, there are counties in the US that spend $1 or less per capita on animal sheltering and rescue.
Rowan, Andrew N., "The Business of Animal Protection" (2013). Animal Abuse, Animal Welfare, and Animal Protection. Paper 21.