WooCommerce custom sale price based on role

woocommerce_custom_sale_price_based_on_user_role.php

<?php
/**
 * Put this into your functions.php of your child-theme or custom plugin
 * you can create the role with wp-cli by running `wp shell` and running the command:
 * add_role('merchant','Merchant',array('read' => true, 'delete_posts' => false) );
 */

/**
 * Step #1: create the field used to store the new sale_price for product_variation and for products
 */
add_action( 'woocommerce_product_after_variable_attributes', 'ran_variation_settings_fields', 10, 3 );
function ran_variation_settings_fields( $loop, $variation_data, $variation ) {
	woocommerce_wp_text_input(
		array(
			'id'          => '_merchant_price[' . $variation->ID . ']',
			'label'       => __( 'Merchant-Price', 'ran' ),
			'desc_tip'    => 'true',
			'description' => __( 'What is the merchant price of this product?', 'ran' ),
			'value'       => get_post_meta( $variation->ID, '_merchant_price', true ),
			'type'        => 'text'
		)
	);
}

add_action( 'woocommerce_product_options_general_product_data', 'ran_add_custom_general_fields' );
function ran_add_custom_general_fields() {
	global $woocommerce, $post;
	echo '<div class="options_group">';
	woocommerce_wp_text_input(
		array(
			'id'          => '_merchant_price[' . $post->ID . ']',
			'label'       => __( 'Merchant-Price', 'ran' ),
			'placeholder' => '',
			'desc_tip'    => 'true',
			'description' => __( 'What is the merchant price of this product?', 'ran' ),
			'value'       => get_post_meta( $post->ID, '_merchant_price', true ),
			'type'        => 'text'
		)
	);
	echo '</div>';
}

/**
 * Step #2: save the merchant value
 */

add_action( 'woocommerce_process_product_meta', 'ran_add_custom_general_fields_save' );
add_action( 'woocommerce_save_product_variation', 'ran_add_custom_general_fields_save', 10, 2 );
function ran_add_custom_general_fields_save( $post_id ) {
	if (!array_key_exists('_merchant_price', $_POST)) return;
	if (!is_array($_POST['_merchant_price'])) return;
	if (!array_key_exists($post_id, $_POST['_merchant_price'])) return;

	$woocommerce_field = $_POST['_merchant_price'][ $post_id ];
	update_post_meta( $post_id, '_merchant_price', esc_attr( $woocommerce_field ) );
}

/**
 * Step #3: define when the new price will be used for discounts
 */
function ran_current_user_is_merchant() {
	$user = wp_get_current_user();
	if ( empty( $user ) ) {
		return false;
	}
	if ( $user->user_email == "alexander@rent-a-ninja.org" || in_array( 'merchant', (array) $user->roles ) ) {
		return true;
	}

	return false;
}

/**
 * Step #4: change the price based on the above function
 */
add_filter( 'woocommerce_product_get_price', 'ran_custom_price', 10, 2 );
add_filter( 'woocommerce_product_variation_get_price', 'ran_custom_price', 10, 2 );

add_filter( 'woocommerce_product_get_sale_price', 'ran_custom_price', 10, 2 );
add_filter( 'woocommerce_product_variation_get_sale_price', 'ran_custom_price', 10, 2 );

add_filter( 'woocommerce_variation_prices_price', 'ran_custom_price', 10, 2 );
add_filter( 'woocommerce_variation_prices_sale_price', 'ran_custom_price', 10, 2 );

function ran_custom_price( $price, $product ) {
	if ( ran_current_user_is_merchant() ) {
		// get the merchant_price and return if available
		$merchant_price = floatval( $product->get_meta( '_merchant_price', true ) );
		if ( $merchant_price ) {
			return $merchant_price;
		}
	}

	return $price;
}


/**
 * Step #5: Make sure the variation price hash is is affected by the function defined in step #3
 */
add_filter( 'woocommerce_get_variation_prices_hash', 'ran_woocommerce_get_variation_prices_hash', 10, 3 );
function ran_woocommerce_get_variation_prices_hash( $price_hash, $product, $for_display ) {
	$price_hash['merchant'] = ran_current_user_is_merchant() ? 'true' : 'false';

	return $price_hash;
}


/**
 * (optional) Step #6: Make the cart look a little nicer
 */
add_filter( 'woocommerce_cart_product_price', 'ran_woocommerce_cart_product_price', 20, 2 );
function ran_woocommerce_cart_product_price( $price_html, $product ) {
	if ( ! $product->get_sale_price() || ! $product->get_regular_price() || $product->get_sale_price() >= $product->get_regular_price() ) {
		return $price_html;
	}

	return 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();
}

/**
 * (optional) Step #7: Show net prices based on the function in step #3
 */
add_filter( 'option_woocommerce_tax_display_shop', 'ran_show_vat' );
add_filter( 'option_woocommerce_tax_display_cart', 'ran_show_vat' );
function ran_show_vat( $value ) {
	if ( is_admin() ) {
		return $value;
	}
	if ( ran_current_user_is_merchant() ) {
		return 'excl';
	}

	return $value;
}

Thank you @xandl for this - I'd managed to get most of the way myself but was stuck with the caching and hash, so you've saved my day!