Is this like in Breaking Bad/Better Call Saul when he would ask for a dollar so then they would then have lawyer/client privilege?
The evidentiary privilege also generally applies in situations where a person is consulting with an attorney and reasonably believes the information exchanged in the consultation is confidential. So in the Breaking Bad episode, Saul didn't really need the dollar to magically "seal" the conversation and have it be treated as confidential.
Interestingly, confidentiality (returning to the ethical rule, as distinguished from the evidentiary privilege) *may* be waived by an attorney to prevent reasonably certain loss of life or substantial bodily harm. Note, the attorney *may* choose to waive confidentiality -- but is not required to!
Same thing with Saul and Walter. Up until that point, they were speaking as hostage and kidnapper. So to distinguish the change in the relationship, he takes a nominal fee. Now they are a lawyer and his clients.
He may have needed it to persuade his clients that the conversation had been magically sealed.