1) Remove the space between $row and ['Price']:
$row['Price']
Not
$row ['Price']
2) Place another set of double quotes at the end and terminate the echo statement with a semicolon. Also it is <br />, not </br>:
" </br>";
You also got a <br> in the index of the post array. Remove it.
Use this:
$query = "SELECT * FROM cars WHERE Cname = '" . $POST['Cname'] . "'";
echo "<b>You have selected the {$POST['Cname']}<br />";
echo "The price is: " . $row['Price'] . "<br />";