Set product sale price programmatically in WooCommerce 3

The hook woocommerce_get_sale_price is deprecated since WooCommerce 3 and replaced by woocommerce_product_get_sale_price.

Also Product displayed prices are cached. When sale price is active, regular price is also active.

Try this instead:

// Generating dynamically the product "regular price"
add_filter( 'woocommerce_product_get_regular_price', 'custom_dynamic_regular_price', 10, 2 );
add_filter( 'woocommerce_product_variation_get_regular_price', 'custom_dynamic_regular_price', 10, 2 );
function custom_dynamic_regular_price( $regular_price, $product ) {
    if( empty($regular_price) || $regular_price == 0 )
        return $product->get_price();
    else
        return $regular_price;
}


// Generating dynamically the product "sale price"
add_filter( 'woocommerce_product_get_sale_price', 'custom_dynamic_sale_price', 10, 2 );
add_filter( 'woocommerce_product_variation_get_sale_price', 'custom_dynamic_sale_price', 10, 2 );
function custom_dynamic_sale_price( $sale_price, $product ) {
    $rate = 0.8;
    if( empty($sale_price) || $sale_price == 0 )
        return $product->get_regular_price() * $rate;
    else
        return $sale_price;
};

// Displayed formatted regular price + sale price
add_filter( 'woocommerce_get_price_html', 'custom_dynamic_sale_price_html', 20, 2 );
function custom_dynamic_sale_price_html( $price_html, $product ) {
    if( $product->is_type('variable') ) return $price_html;

    $price_html = wc_format_sale_price( wc_get_price_to_display( $product, array( 'price' => $product->get_regular_price() ) ), wc_get_price_to_display(  $product, array( 'price' => $product->get_sale_price() ) ) ) . $product->get_price_suffix();

    return $price_html;
}

Code goes in function.php file of your active child theme (active theme).

Tested and works on single product, shop, product category and tag archive pages.

The continuation in:
Wrong Woocommerce cart item price after setting programmatically product sale price