Synonyns in this position:
They still bought it, although, it was expensive.
They still bought it, however, it was expensive.
They're not synonymous here to me. The second doesn't work, IMO, and I'd delete the second comma in the first.
Synonyns in this position:
It was expensive. They still bought it, (al)though.
It was expensive. They still bought it, however.
These wok, although I'd say that 'though' is the form to use in the first.
