Different companies sell flakes/crystals from different sources. Magnesium oil is of course the least cost effective way to add magnesium chloride to your bath, but there are a couple companies with decent pricing on the flakes. Surprisingly, the Ancient Minerals bath flakes that Dr. Sircus recommends are actually very reasonably priced... if you order 32 pounds of the ancient minerals flakes, it costs about $150. For the top of the line stuff, I don't think that's too bad.
Also, magnesium oil CAN be made by mixing nigari into water, but that is not how they are all made (or even most of them as far as I can tell). Nigari only happens to be one source for magnesium oil. The dead sea, pacific ocean, great salt lake, underground seabeds, etc, etc... all produce different qualities.