Convert the calculation of difference from a value into text and specify a number format, then add an ampersand between the text and the calculation to join the two together:
,"You are over budget by " & Text(F3-F2,"#0.00"),
Repeat for the value if false.
Using TEXT() gives you control of how the number will be displayed. ,"You are over budget by " & F3-F2, will work but the number format is not settable.