How much are you willing to pay someone to handle incredibly venomous snakes, milk them, use that venom to create a vaccine that makes a horse or cow develop antibodies for it, administer that to said animals for a long time, extract a ton of blood from those animals, extract the antibodies from the blood, create a nifty injectable out of those antibodies and ship it to you?
A lot goes into making antivenom.
Also, the US only has one FDA approved and licenced antivenom for pit vipers (rattle snakes, copperheads and water moccasins) which is CroFab. They can charge whatever the hell they want, since they're the only game in town, and when it comes to life or death situations people are often willing to pay anything.
CroFab could be swindlers, but there is a significant legitimate expense in making antivenoms, and people aren't bitten by venomous snakes often enough to produce quantities large enough to drive costs down and encourage competition in our awful market-ish based healthcare system where the people who buy aren't the people who pay.