显示DB的价格

The user goes change the nm_peca, will select tipo_preco and then find the price in MySQL database to complete the textbox vlr_peca.

What do I need to do to get the value of products?

<?php>
$query = mysql_query("SELECT id, nm_peca, vlr_original, 
                             vlr_paralelo, fabricante 
                        FROM peca 
                       ORDER BY nm_peca");
<select id="nm_peca" name="nm_peca" 
      title="Selecione um status." 
      class="form-control" size="1">
    <option value="">Status</option>
    <option value="N/A">N/A</option>
    <option value="S">S - Substituir</option>
</select>
    </div>
</div>

<select id="tipo_preco" name="tipo_preco"  
        class="form-control" size="1">
    <option value="">Tipo</option>
    <option value="Peça Original">Original</option>
    <option value="Peça Paralela">Paralela</option>
</select>
    </div>
</div>

<div class="col-md-1">
<div class="input-group btn-group">
    <input type="text" value="????????" name="vlr_peca" 
           class="form-control" readonly>
</div>
</div>

Well your code is a little confusing because what you really want is not clear to me, maybe it doesn't help that it's in a different language. What is the $query for? You are not using this in the code, so you'll need to do this to access it:

$rows = array();
while ($row = mysql_fetch_assoc($query)) {
    $rows[] = $row;
}

print '<pre>'. print_r($rows, true) . '</pre>';

Then you'll need to use an ajax request OR setup javascript vars on the front end to make changes. I'm just going to go ahead and use the latter since it's more illustrative.

So I would add the cdn of jQuery which is https://code.jquery.com/jquery-2.1.4.min.js to the top of your file in a tag like:

<script type="text/javascript" src="https://code.jquery.com/jquery-2.1.4.min.js"></script>

Then you'll need to add an input id to that input tag like <input id="dynamic-variable" type="text" value="????????" name="vlr_peca"> and then use something like:

<script type="text/javascript">
$(document).ready(function() {
   $('#tipo_preco').change(function() {
     if ($(this).val() == 'Peça Original') {
       $('#dynamic-variable').val('$5,000,000,000.00');
     } else if ($(this).val() == 'Peça Paralela') {
       $('#dynamic-variable').val('$300 billion trillion dollars');
     } else {
       $('#dynamic-variable').val('Bro.  Choose one.');
     }
   });
});
</script>

I'm not going to program this whole thing for you since I don't know what kind of data you are getting from your DB, but this should be good enough to get you started. You need to be able to access the data in the mysql query, and you may just want to use the <?= $db_variable ?> or <?php echo $db_variable?> in your javascript where I have the amount variables. This should be good enough to get you started. Read up on php mysql_query and jQuery

AJAX If you were to do this via AJAX, you would need a second page that accesses the database, you'd have to $_GET or $_POST variables and then print back the amount.

So your JavaScript would be something like this:

$.get('/the/page.php?tipo_preco=' + $('#tipo_preco').val(), function(response) {
    if (response != '') {
       $('#dynamic-variable').val(response);
    }
});

And your second page would be something like this:

<?php 
// code that sets up the database
// .... //
// now your code
$selected_value = $_GET['tipo_preco'];
$result = mysql_query("SELECT price FROM table WHERE column_val = '" . mysql_real_escape_string($selected_value) . "'");
$row = mysql_fetch_assoc($result); // assuming you're only fetching one row
print $row['price'];
?>

I would suggest you just go with the first suggestion, it's faster, less moving parts.