Numerical sorting works on numbers. Any function you would like to use for this would at some point have to transform '15600>18200' into numerical data. Since this transformation would be inevitable internally, I recommend you change the representation of total income altogether, as this would make more sense semantically as well.
If it would be too time consuming to do this manually, the process can probably be automised, but the easiest way to go about doing that will depend on how you are storing and manipulating the dataset.