This thread really follows on from an earlier one entitled 'dumb as a post', which developed into comments about regional use of an idiom. I thought I'd toss this in the air and see how it came down!
When someone uses an idiom in the wrong context, it causes a certain astonishment in the listener and a desire to say "WHAT?" That is how I'd feel if you thanked me for my help by saying "Thanks a bunch".
In my experience, 'thanks a bunch' definitely does not ever mean a sincere thank you. It is a purely sarcastic expression that I would only use if, for example, a friend had let me down in some way.
All the other genuine expressions of thanks can also be used ironically (eg: "Thank you very much" when someone spills his drink over you), but this would be understood from the context or the speaker's tone and face.
I may be alone here, but am confident this is correct, at least in England. Unfortunately you do hear some native English speakers using this expression wrongly, but that doesn't make it 'right'. Is this the same in other regions, I wonder?
"Thanks a bunch" definitely CAN be used in a sarcastic manner, and maybe is used even more often in a sarcastic manner, but I'd say it can definitely be used sincerely.
From my perch here in Toronto, I agree with Vidor. Thanks a bunch is usually used ironically, but it could also be jocular in a folksy or mock-folksy way. You'd have to rely on context and intonation to be certain.
I don't think I've ever heard it except with one or the other of these connotations. It would never be a simple synonym for Thank you very much.
Because it is used regularly in a sarcastic manner, it is a phrase that should be used with caution by some learners- if they get their intonation wrong, they could easily be misunderstood.
All about tone, really. Straightforward phrases like "Thank you very much" become sarcastic if said in a sarcastic tone.
Not all learners have native speaker level control of and sensitivity to tone.