<script src="https://code.jquery.com/jquery-3.6.0.min.js" integrity="sha256-/xUj+3OJU5yExlq6GSYGSHk7tPXikynS7ogEvDej/m4=" crossorigin="anonymous"></script>

<?php
  $euro1 = 20;
  $euro2 = 40;
  $euro3 = 60;
  $euro4 = 80;
?>

<select id="euro">
  <option value="<?php echo $euro1; ?>">1 Euro Kaç TL</option>
  <option value="<?php echo $euro2; ?>">2 Euro Kaç TL</option>
  <option value="<?php echo $euro3; ?>">3 Euro Kaç TL</option>
  <option value="<?php echo $euro4; ?>">4 Euro Kaç TL</option>
</select>

Sonuc: <span id="result"><?php echo $euro1; ?></span>

<script>
  $("#euro").on('change', function () {
    $("#result").html(this.value);
  });
</script>