It's an unusually missing feature, but the report builder is only able to show totals on summarized reports, but not on unsummarized.
This means for many Deal or Company based reports in which you need to show the individual records, and so need to use an unsummarized report, you can't also have a total field at the bottom.
This is particularly useful when Deal values are involved!
I've tried a few variations and it's just not possible, and after speaking to support they guessed it was an unintentional UI issue to be looked at, and raised it internally.