Change the input type to number
<label for="income-1">Income 1</label>
<input type="number" id="income-1" ng-model="user_data.income_1" />
<label for="income-2">Income 2</label>
<input type="number" id="income-2" ng-model="user_data.income_2" />
and it will convert to numbers automatically.
In a more complex situation (for example you need input type to be text for some reason), you can easily build directives to take care of the conversion, see e.g. Input model changes from Integer to String when changed