wordpress use single ajax in place of multiple ajax requests in a smarter way

In wordpress I have database tables like this

product name

id product_name

1   A
2   B
3   C
4   D

product_buying_price

id product_name  product_buying_price
1   A                               10
2   B                           12
3   C                               15
4   D                               18

product_selling_price

id product_name product_selling_price
1   A                               12
2   B                           13
3   C                               19
4   D                               23

So I have a dropdown like this

<select name="product_name" id="product_name">
    <option value="">Products</option>
    <option value="A">A</option>
    <option value="B">B</option>
    <option value="C">C</option>
    <option value="D">D</option>
</select>

and I want to show the values for product buying price and product selling price inside a table like this

<table>
    <tr>
        <td>
            <select name="product_name" id="product_name">
                <option value="">Products</option>
                <option value="A">A</option>
                <option value="B">B</option>
                <option value="C">C</option>
                <option value="D">D</option>
            </select>
        </td>
        <td class="product_buying_price"></td>
        <td class="product_selling_price"></td>
    </tr>
</table>

Like this I have more around 3-4 such fields and I want to get those values for the product.

In the js file I have made the ajax like this

$('body').on('change','#product_name', function() {
    var selected = $(this).val;

    //Get product buying price
    $.post( test.ajaxUrl, { 'selected' : selected,  'action' : 'get_product_buying_price' }, function(data){
        data = $.parseJSON(data);
        selected.find('td.product_buying_price').html(data);
    });

    //Get product selling price
    $.post( test.ajaxUrl, { 'selected' : selected,  'action' : 'get_product_selling_price' }, function(data){
        data = $.parseJSON(data);
        selected.find('td.product_selling_price').html(data);
    }); 
});

Inside function I have the function like this

function get_product_buying_price() {
    global $wpdb;
    $selected_product = $_POST['selected'];
    $get_product_price = $wpdb->get_row("SELECT `product_buying_price` FROM `product_buying_price` WHERE `product_name` = '.$selected_product.' ");
    $product_buying_price = $get_product_price->product_buying_price;
    echo json_encode($product_buying_price);
    exit;
}

function get_product_selling_price() {
    global $wpdb;
    $selected_product = $_POST['selected'];
    $get_product_price = $wpdb->get_row("SELECT `product_selling_price` FROM `product_selling_price` WHERE `product_name` = '.$selected_product.' ");
    $product_selling_price = $get_product_price->product_selling_price;
    echo json_encode($product_selling_price);
    exit;
}

Here its working fine. But don't you think that doing multiple ajax request on one click makes it slow? I have around 3-4 request for one change. So can someone tell me some smarter way to achive this? Any help and suggestions will be really appreciable. Thanks.