In the following, assume the variable valueToAdd is a string value representation of the number you want to add and total is a float value. Using a string, you can check for an existing decimal with the find() method. Given this information, you would use the following to set the total amount after the addtion.
set total to total + (find(valueToAdd, ".") = -1 ? float(valueToAdd) / 100 : float(valueToAdd))
What this does is, first checks if a decimal is present in the valueToAdd (find(valueToAdd, ".") = -1). If the result of the find is -1, no decimal is found so we divide by 100 (float(valueToAdd) / 100). If the result of the find is > -1 the string already contains a decimal so, no need to divide.
To get more information about the statement, read the ?: section of the expressions tutorial here:
https://www.scirra.com/tutorials/77/nat ... uct-2#h2a0
I hope this gets you what you need and good luck with your project.