marijuana testing process
As a starting point, THC testing is advertised online for the following prices:

--Green Leaf Lab: $75

--Pharm Labs : $75

These companies both use HPLC. Without looking anything up, I'd guess that: The machine is around $100,000. It measures a sample every 30 minutes. (There might be modern marvels of engineering that run several samples in parallel and run way faster - idk.) Solvents and consumables are cheap. Maintaining the HPLC is expensive. You need a trained technician to prepare samples, run the analysis, and troubleshoot. That's expensive. You also need someone to deal with customers, which can be time-consuming as well.

You are not the only person thinking about THC testing. Google finds a handful of articles on what a business opportunity it is, and equipment manufacturers are ready to sell you stuff.

Good luck!