sed - but tamen - however, nevertheless

What does these words then mean when placed together? Placing a "but" in front of "however" seems unnecessary to me.

I often see this at the end of "unfinished" phrases, in conversational Esperanto. Most cases are discussions, where the standpoint of the speaker is already known:

Mi komprenas kion vi diras sed tamen... Estas bela ideo sed tamen... Mi scias kiom da kostoj ni jam havas, sed tamen...

I've been translating this to "but still" in my head. Is that correct?