iWork Numbers evaluate string
You can use the VALUE() function for that. Its only argument is the cell that you want to evaluate

From the documentation:


  The VALUE function returns a number value, even if the argument is formatted as text. This function is included for compatibility with tables imported from other spreadsheet apps.
  
  VALUE(source-string)
  source-string: Any value.
  
  Notes
  You’ll never need to use the VALUE function in a new table, because numbers in text are automatically converted for you.
  
  Only the formatted text is converted. For example, if you type the string $100.001 into a cell, the default format will display only two decimals ($100.00). If VALUE refers to this cell, it will return 100, the value of the formatted text, not 100.001.
  
  If the argument can’t be returned as a number value (does not contain a number), the function returns an error.
  
  Examples
  =VALUE(“22”) returns the number 22.
  
  =VALUE(RIGHT(“The year 1953”, 2)) returns the number 53.