In your code it looks like you are using the same input for the value of both search criteria:
This will not give you any results because you are saying, 'give me the values from this column where S_dhpc_Prezzo is less then or equal to the value in the db, AND where S_dhpc_Prezzo is greater than or equal to the value'. This does not make sense and will not yield any results for you. Instead you must have two separate form elements, one for the minimum value and another for the max value. Use the max value one with the <= criteria, and the minimum value one for the >= criteria.