mysqli:query() returns a mysqli_result object, which cannot be serialized into a string.
You need to fetch the results from the object. Here's how to do it.
Fetch a single row from the result and then access column index 0 or using an associative key. Use the null-coalescing operator in case no rows are present in the result.
$result = $con->query($tourquery); // or mysqli_query($con, $tourquery);
$tourresult = $result->fetch_array()[0] ?? '';
// OR
$tourresult = $result->fetch_array()['roomprice'] ?? '';
echo '<strong>Per room amount: </strong>'.$tourresult;
Use foreach loop to iterate over the result and fetch each row one by one. You can access each column using the column name as an array index.
$result = $con->query($tourquery); // or mysqli_query($con, $tourquery);
foreach($result as $row) {
echo '<strong>Per room amount: </strong>'.$row['roomprice'];
}