That’s a little steep for the number of flex sensors I’d want. Other options I’m thinking of, one after another, commission of making the thing I’m concerned about flexing, hollow. That might not be the best idea. Conductive paint under varnish might give a good way to identify a fracture though.
How about an external cable, kept trait with springs, that holds a magnet in the middle of the arm. In the arm itself wild be a hall effect sensor?
If the arm flexes too much, the magnet changes position. Of course, this places more strain on the whole assembly.