Does anyone know the wholesale price of worm/parasite external drops for dogs/cats?

I live on a farm in eastern Canada & am constantly having cats dumped on me. Treatment for 3 cats from a local vet is over $100.00 . I have 30 cats. No I am not a hoarder. I believe though that we are being ripped off & greatly being taken advantage of by those who claim to care but seem to only want the cash. Its over a $100.00 also to have a cat euthanized. I don’t have a problem with someone making a living but it seems that these practices promote more abandoned animals than help.