For healthy food on cheap airlines you have to invest some time in planning and focus on picking stuff before you board...
- for people on protein diets (like me)
- get salat+meet combo + shake+nuts @McDonalds /// probably only thing worth buying in McDonalds anyway
- for all
- take lots of local fruits (in Germany this would be apples)
NOTE: - on smaller/regional airports chices are minimum and prices might get much higher