erm actually
what they're saying is that you can get a breakfast that would normally be worth $10, except for free.(unless there's some sort of extra details I didn't read about, but they'd still be irrelevant because of the following)
The joke is that the phrase "free $10 breakfast" sounds self-contradictory because it can't be both free and $10 (as those are mutually exclusive terms), even though that isn't ACTUALLY what they meant.