Simple budget program
Here are the few things I've noticed after a first glance on the program:


import os is not used, you can remove it
() after the class Application can be omitted
keep two blank lines between the imports and the class definition, after the class code and the if __name__ == '__main__' line
variable naming - there are some poor variable names like x or k - consider renaming them to something more descriptive
the following code block can be removed (unless you have it there as a stub for future development):

else:
    return

the class itself feels overloaded - this may be because it does two different not-directly related things at the same time - prompting the user and making budget calculations - since you are going to continue to develop the class - make sure Application is not becoming a God Object
the class and its methods would benefit from having detailed documentation strings


As a side note, take a look at the The Architecture of Open Source Applications project - it is really pure informational gold in terms of designing, thinking and development of your software project.