How to add Extra Input Field to Wordpress Posts?

Registering Price meta box for your blog post you can use this code.

add_action( 'add_meta_boxes', 'price_meta_box_add' );
function price_meta_box_add() {
    add_meta_box( 'price-meta-box-id', 'Put Price Here!', 'price_meta_box_cb', 'post', 'side', 'high' );
}
function price_meta_box_cb() {
    wp_nonce_field( basename( __FILE__ ), 'price_meta_box_nonce' );
    $value = get_post_meta(get_the_ID(), 'price_key', true);
    $html = '<label>Price: </label><input type="text" name="price" value="'.$value.'"/>';
    echo $html;
}
add_action( 'save_post', 'price_meta_box_save' );
function price_meta_box_save( $post_id ){
    if( defined( 'DOING_AUTOSAVE' ) && DOING_AUTOSAVE ) return;
    if ( !isset( $_POST['price_meta_box_nonce'] ) || !wp_verify_nonce( $_POST['price_meta_box_nonce'], basename( __FILE__ ) ) ) return;
    if( !current_user_can( 'edit_post' ) ) return;
    if( isset( $_POST['price'] ) )
        update_post_meta( $post_id, 'price_key', esc_attr( $_POST['price'], $allowed ) );
}

to output that field in your post page single.php you must use this code to output. Make sure you put this code in loop if(have_post) while loop ...

<?php get_post_meta(get_the_ID(), 'price_key', true); ?>

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