I use Plantation Blackstrap, that's what is available mostly in the stores where I live... I have seen extra-large bottles of it also in some places, not sure what the cost was.
Anyways, with a blackstrap molasses especially it is a more complex mixture than just "sugar", which is a refined chemical substance (sucrose). Blackstrap is the final by-product of sugar-refining, left over from the last (3rd) boiling of cane or beet juice to extract the sucrose. This leaves behind a product which is more bitter, as it has a significantly higher concentration of minerals and less, but nonetheless still some sugars (mainly sucrose, fructose and glucose) and other substances (including some vitamins, organic acids).
Blackstrap generally has a good K content, aside from minerals (Iron, Calcium, Zinc, Copper, etc) which microbes also require for growth.