Hi Marcel,
If you want to use a question with a text answer type to drive a formula then you'll need to provide a restricted range of possible responses and then map those reponse onto numeric values for use in the formula. It's not feasible to do it for a text question with a free form answer as far as I know.
To achieve this you need to edit the question and do the following:
a) set the 'include in cost' attribute to 'custom'
b) set the 'acceptable values' to 'list of choices'
c) when you add the choices at the bottom of the screen you should see a section that allows to map each one to a numeric value for use in formulas
d) refer to the numeric value in the if statement in the formula (although maybe you don't need an if statement when you already have a numeric value to work with, it's not really clear from your post)
Thanks