People are skeptical because they are comparatively so cheap. But, think of how much money pharmaceutical companies have to recover off their products for research, testing, advertising, packaging, etc.
I mean, what makes a bottle of 1000 aspirin $10 dollars? Is the process by which the chemicals and tablets are made so different than the one used to make ox? The answer is no.
Trust me, any underground lab which pays nothing for testing, licensing, etc. is still going to make a killing without all the overhead and red tape that "legitimate" labs will pay.