Are you sure that these precursors aren't just expensive because so many of the compounds that go into them, or which come from them, are controlled substances?
There are a lot of bulk reactants that I'd like to buy for chemistry experiments, but can't (because they're controlled, because they can be used to make illegal things), so I have to make them myself from even earlier precursor chemicals; this adds (via labor costs) to the cost of making the final product. Presumably, even big producers like Sigma are in the same situation.
Sigma sells cocaine, heroin, and LSD, for example, nevermind precursors. They won't sell them to you, but they will sell them to organizations with the correct permissions to buy them. Regulatory compliance might add to the cost, but for a company like Sigma I'd expect doing things in-house to lower costs not raise them.
There are a lot of bulk reactants that I'd like to buy for chemistry experiments, but can't (because they're controlled, because they can be used to make illegal things), so I have to make them myself from even earlier precursor chemicals; this adds (via labor costs) to the cost of making the final product. Presumably, even big producers like Sigma are in the same situation.