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TruWalley, my opinion springs from mostly guesswork and hearsay but, as I understand it, yeah, it's very cheap to manufacture. Because it is so powerful, a little goes a long way -- it's measured in micrograms whereas most PKs are measured in milligrams. So transportation and storage are more economical, too. With all the negative publicity the past couple of years, I think people have become more wary of the danger, but not too long ago fentanyl's deadly power was actually an attraction for some people into heavy recreational use, and they were glad to hear it had been used as a cut -- they believed they could handle it, of course.
The only way to know what, if anything, was added to the pills you got is to test them--but please, not with your own body! The only question that will answer is whether they were fatal to you--because even if you survive, you still won't know what they actually are!
As other people mentioned, you can buy test kits on eBay, Amazon, etc. Apparently, some are just for fentanyl; there are also some decent little kits that test for a variety of substances. They are not very expensive, with some running well under $50 including shipping.