You can do it with a Macro
Right-click on the sheet tab and select "View Code"
Copy & paste in the following:
Private Sub Worksheet_Change(ByVal Target As Range)
If UCase(Target.FormulaR1C1) = "X" Then
Target.FormulaR1C1 = "$5.00"
Close out of the code window.
Now, if you type in 'x' in any cell, it will turn into "$5.00"