And, yes, there was a reason they didn't mention what the experiment actually entailed until he got there. The company running the experiment, with the delightfully evil-sounding name Pharmaco, was testing a drug to help with the healing of skin and muscle tissue. What they didn't mention in the ad is that to test "healing" his skin, they would have to "wound" his skin. A few football-shaped chunks out of the backs of his arms and a seven-day hospital stay later, and he found himself $2,000 richer.
He would have held the money in his arms but, you know, mangled.
This is when Rodriguez figured out that the medical studies that paid the most were high-paying for a reason -- they tended to hurt. Still, money was money, so he came back for more, signing up for studies that promised even longer stays (and presumably more awful side effects).
To fund El Mariachi directly, he signed up for a one-month drug trial that paid $3,000 but put him through Draconian scheduling (his days were planned out to the minute, and showing up late to things like meals or his daily blood draws would cost him $25 an occurrence). Even more fun, he was required to poop into clear Tupperware, place it in a fridge alongside the other patients' samples and discuss his poop's consistency with a drug counselor.
It only cost $7,000 and a lifetime of human dignity.
It would be worth it, in the end. Though the film didn't make much money, the right people noticed it. This enabled Rodriguez to more or less remake it as Desperado, launching the English-speaking careers of Salma Hayek and Antonio Banderas in the process. Fifteen years later, Rodriguez is still making movies ... and he still has his medical testing scars to remind him where he came from.