How do you figure out the dissociation energy for ions?
If you are trying to find the heterolytic dissociation energy for a gaseous $\ce{KF}$ molecule then as a rough estimate you can model the ions as point charges and calculate the energy required to separate them from their equilibrium separation (the bond length) to infinity, using Coulomb's Law: $$E = -\int_{2.17\times10^{-10}}^\infty\frac{Qq}{4\pi\epsilon_0r^2}dr$$ 

Note that modelling the ions as point charges is a major simplification and so this is probably not a very accurate estimate. Unfortunately I can't find any data for the actual energy so I can't say how accurate this is.