But everytime you slightly change the contents, Dropbox will have to resend the entire container - thats one of the features of good encryption
As far as I can tell from my own usage, the only portion that is uploaded is the changed portion. I have a 1GB container currently synced and when I change files in that container, I can assure you that it doesn't upload 1GB of information. It appears to only transfer the delta.
I don't know how the two are supposed to combine, but the prediction is that for a truecrypt disk image, any change in the contents produces a very different disk image all over - so transferring "the delta" is still a much larger amount than you'd expect.
Although, if this were the case it would also have to rewrite the disk image a lot and that might be quite bad for just local use.