They still bought it, although, it was expensive.
They still bought it, however, it was expensive.
Synonyns in this position:
It was expensive. They still bought it, (al)though.
It was expensive. They still bought it, however.
Although it was expensive, they still bought it.
However it was expensive, they still bought it. ungrammy
Within a given sentence, the word 'although' can modify A or B
[although A] + B
A + [B although]
Within a given sentence, the word 'however' can modify B
A + [however + B]
A + [B however]
Hypothesis: Both 'although' and 'however' function as conjunctions, but 'however' functions in a way similar to that of a pronoun: it needs a referent. Placing 'however' before its referent produces a result kind of like:
?He's the guy I told you about, John. semantics
wherein the intended meaning is 'He' and 'John' are the same person (co-referent), but the order of the pronouns tells us that 'He' and 'John' are not the same person. The reason being that pronouns need their referents to come before them, so as they have something to anchor to, to refer to.
In that 'He' to refer to 'John', we need to place 'John' before
John Smith is his name. He's the guy I told you about.
It's that kind of distribution, I believe, that makes 'however' similar to a pronoun. That is, "However it was expensive, they still bought it" is ungrammy 'cause 'however' needs to come after A and before B:
A + [however + B] (wherein "A" is a referential sentence)
It's the order, not the semantics, per se, that's important.
In short, 'however' needs a referent. 'although' doesn't.