WooCommerce - Display only Sale Price https://www.damiencarbery.com/2018/04/woocommerce-display-only-sale-price/

regular-and-sale-price.html

<span class="price">
  <del><span class="woocommerce-Price-amount amount">
    <span class="woocommerce-Price-currencySymbol">&euro;</span>65.00
  </span></del>
  <ins><span class="woocommerce-Price-amount amount">
    <span class="woocommerce-Price-currencySymbol">&euro;</span>55.00
  </span></ins>
</span>

sale-price-with-rrp-by-category-or-product.php

<?php
/*
Plugin Name: Show Sale Price and RRP - per product or category
Plugin URI: https://www.damiencarbery.com/2018/04/woocommerce-display-only-sale-price/
Description: Display discounted or sale price with the RRP under it. Limit to specified products and categories.
Author: Damien Carbery
Author URI: https://www.damiencarbery.com
Version: 0.2
*/


add_filter( 'woocommerce_format_sale_price', 'dcwd_sale_price', 20, 3 );
function dcwd_sale_price( $price, $regular_price, $sale_price ) {
	// Display regular price when viewing these products or categories.
	$products = array( 27, 28 );
	$categories = array( 'accessories', 'hoodies' );

	$show_regular_price = false;

	// Include products in a category archive and in related products section.
	$product_id = get_the_ID();
	if ( in_array( $product_id, $products ) ) {
		$show_regular_price = true;
	}
	
	// Check whether current category is in the list.
	if ( is_product_category() ) {
		$queried_object = get_queried_object();
		$category_slug = $queried_object->slug;
		
		if ( in_array( $category_slug, $categories ) ) {
			$show_regular_price = true;
		}
	}

	if ( $show_regular_price ) {
		return sprintf( '<span class="sale_price">%s</span> <span class="rrp">RRP: %s</span>',
					wc_price( $sale_price ), wc_price( $regular_price ) );
	}
	else {
		return $price;
	}
}

sale-price-with-rrp.php

<?php
/*
Plugin Name: Show Sale Price and RRP
Plugin URI: https://www.damiencarbery.com/2018/04/woocommerce-display-only-sale-price/
Description: Display discounted or sale price with the RRP under it.
Author: Damien Carbery
Author URI: https://www.damiencarbery.com
Version: 0.1
*/


add_filter( 'woocommerce_format_sale_price', 'dcwd_sale_price', 20, 3 );
function dcwd_sale_price( $price, $regular_price, $sale_price ) {
	return sprintf( '<span class="sale_price">%s</span> <span class="rrp">RRP: %s</span>',
                 wc_price( $sale_price ), wc_price( $regular_price ) );
}

sale-price.html

<span class="price">
  <span class="woocommerce-Price-amount amount">
    <span class="woocommerce-Price-currencySymbol">&euro;</span>55.00
  </span>
</span>

show-only-sale-price-variable.php

<?php
/*
Plugin Name: Show Only Sale Price (variable products)
Plugin URI: https://www.damiencarbery.com/2018/04/woocommerce-display-only-sale-price/
Description: Display only the reduced price when a product is discounted or on sale.
Author: Damien Carbery
Author URI: https://www.damiencarbery.com
Version: 0.1
*/


add_filter( 'woocommerce_variable_price_html', 'dcwd_variable_price', 10, 2 );
function dcwd_variable_price( $price_html, $product ) {
	if ( $product->is_on_sale() ) {
		$prices = $product->get_variation_prices( true ); 
		$min_price = current( $prices['price'] );
		$price = wc_price( $min_price ); 
		return 'From: ' . $price;
	}
	return $price_html;
}

show-only-sale-price.php

<?php
/*
Plugin Name: Show Only Sale Price
Plugin URI: https://www.damiencarbery.com/2018/04/woocommerce-display-only-sale-price/
Description: Display only the reduced price when a product is discounted or on sale.
Author: Damien Carbery
Author URI: https://www.damiencarbery.com
Version: 0.2
*/


// For simple products.
add_filter( 'woocommerce_format_sale_price', 'dcwd_sale_price', 20, 3 );
function dcwd_sale_price( $price, $regular_price, $sale_price ) {
    /*global $product;
    // Show regular and sale price for specified product IDs
    if ( in_array( $product->get_id(), array( 1, 2, 3, 28 ) ) ) {
        return $price;
    }
    // Show regular and sale price if product is in any of the specified categories.
    if ( has_term( array( 'hoodies', 'accessories', 'tshirts' ), 'product_cat', $product->get_id() ) ) {
        return $price;
    }*/
	
	return wc_price( $sale_price );
}


// For variable products.
add_filter( 'woocommerce_variable_price_html', 'dcwd_variable_price', 10, 2 );
function dcwd_variable_price( $price_html, $product ) {
	if ( $product->is_on_sale() ) {
        /*// Show regular and sale price for specified product IDs
        if ( in_array( $product->get_id(), array( 1, 2, 3, 7932 ) ) ) {
            return $price_html;
        }
        // Show regular and sale price if product is in any of the specified categories.
        if ( has_term( array( 'hoodies', 'accessories', 'tshirts' ), 'product_cat', $product->get_id() ) ) {
            return $price_html;
        }*/

		$prices = $product->get_variation_prices( true ); 
		$min_price = current( $prices['price'] );
		$price = wc_price( $min_price ); 
		return 'From: ' . $price;
	}
	return $price_html;
}

For anyone else interested int the code that I wrote for Lyse:
Show regular price in cart and checkout when using WooCommerce Wholesale Prices
https://www.damiencarbery.com/2019/07/show-regular-price-in-cart-and-checkout-when-using-woocommerce-wholesale-prices/