I believe they got some pharmacist to create the drug. No big company wants the liability. And the prison system does not want the liability (nor the PR issues) from using vet drugs.
If the prisoner is deemed to have "excessively suffered", his family's lawyers will be looking to sue everybody, including the pharmacist. The pharmacist's liability insurance does not cover manufacture of execution drugs. The pharmacist also risks harassment from Leftists and BLM (possibly including having his home firebombed) if his identity gets revealed.
Under such circumstances, what would YOU charge, to compensate for the risks?
What efforts did the perp take to make sure that the mother, father, and two young daughters were dead (from repeated beatings) before setting their house ablaze?