codice:
<form method="post" name="form1" id="form1">
    <table align="center">
      <tr valign="baseline">
        <td nowrap="nowrap" align="right">Cognome:</td>
        <td><input type="text" name="Cognome" size="32" /></td>
      </tr>
      <tr valign="baseline">
        <td nowrap="nowrap" align="right">Nome:</td>
        <td><input type="text" name="Nome" size="32" /></td>
      </tr>
      <tr valign="baseline">
        <td nowrap="nowrap" align="right">Card:</td>
        <td><input type="text" name="card" size="32" /></td>
      </tr>
      <tr valign="baseline">
        
        <td>
<input name="crediti" id="crediti" placeholder="Valore da moltiplicare"
 onKeyup="document.querySelector('#result').value=Number(this.value)*Number(document.querySelector('#costomensa').dataset.price)"><br></td>

      <tr valign="baseline">
        <td nowrap="nowrap" align="right">mensa:</td>
        <td><select name="costomensa" id="costomensa">
          <option value="1" data-price="3.5">3,5 - lettera</option>
          <option value="2" data-price="5">5 - allega</option>
          <option value="3" data-price="2">2 - Sura</option>
        </select></td>
      </tr>
      <
          <input type="submit" value="Aggiorna" />
        </p>
        <p>&nbsp;</p>
        </td>
      </tr>
    </table>
    
  </form>

Nota che nel tag select, anzi nei tags option, ho usato un data-attribute che contiene il prezzo espresso usando il punto come separatore dei decimali.