Even within animal testing, the cost and convenience of animal testing could vary tremendously. Working in animal testing intrinsically means running a death-camp for mice. The mice aren't are barely even raw materials, they're basically stationery, more treated more like a mobile pH test-strip then a living creature.

There are ways that animal testing could be made far more humane (like birth control to prevent the kind of overbreeding that demands mass culling), but those, themselves, would be costly.

And in order to test cures, you must first inflict the animals with the disease. How many do you need? Are you sure? Because waiting to produce more later would also create costs. Cheaper just to breed a huge batch now and then cull the surplus.

... so how many extra do you want?