And I doubt it would be easier to get either since they're unlikely to hand out an unlimited quantity to anyone who pays for it even if it were legal.
2. I think some drugs would become cheaper, and other drugs would become more expensive. Drugs with limited supply would become cheaper as more production/supply chains open up. Think cocaine, heroin, and some pharmaceuticals without large overseas labs. There are inefficiencies in the illegal drug trade that legal trade could smooth over as well: better payment methods, more open price negotiations, legal settlement for disputes, reduced employee churn from violence/arrest, reduced bribes/corruption related charges.
3. Regardless of the price of drugs, the overall price would be smaller, as illegal drug use has huge externalized costs. Everything from corrupt governments to increased health care utilization to wasted law enforcement resources to destroyed human capital.
4. The existence of an available quality tested market would force the black market to compete more on quality.
Tranq is cheaper because it is available for animals, and not 'seizing and destroying something while putting the people who possess it in prison' is a great market force for lowering prices.
Ergo, don't criminalize heroin and tranq stops appearing in it.
We already see this with alcohol. Most liquor is tarted-up industrial ethanol, delivered by train tankers to the bottling plants, with some marketing and branding wrapped around it. Industrial ethanol is very inexpensive to produce, around $1/liter; the price of consumption alcohol is downstream of excise taxes and branding.
Cost is not a relevant factor. We can produce all of these chemicals for so little that it is a rounding error. This is a matter of policy.