Basically it is about the same as a -125/-125 hold. Books get away with it because people are stupid enough to bet it. The SuperBowl is like Black Friday to the books. Especially in Vegas, it is the time that those that don't make alot or dont take a lot of action take a lot of action and at extremely high holds.
If you really want to be scared start figuring out the holds on the props where you have multiple possibilites like... who will score first td, who will be MVP etc. Those holods I best are close to 20-25%, while this one is about 10.