I’m really intrigued by your use of “that’s”. I can see that it is used as a possessive analogous to “whose”. I never thought about that, but it is a bit weird that English doesn’t have a possessive for “that” and “which” but does have one for “who” while strictly enforcing the exclusive use of one of the three for different subjects.
Is that a regionalism, a new development in standard English or a neologism on your part? For what it’s worth, I would have felt forced to write the much more involved “a speculation currency the only real use of which is buying heroin” but Merriam Webster says you can use “whose” in this case as well.
