"change product prices via a hook in woocommerce 3+" Code Answer

Update (December 2020)

• 2 code versions for themes and plugins (works in Woocommerce 3.3.x too)
  
• Cached variations prices in Woocommerce 3 (Update and addition):
  Now using woocommerce_get_variation_prices_hash filter hook much more efficient, instead of wc_delete_product_transients()… See this related thread
• Added product price filter widget hooks (see at the end).

1) Plugin version with a constructor function:

The hooks that you are using are deprecated in WooCommerce 3+

To make it work for all products prices, including variations prices, you should use this:

## The following goes inside the constructor ##

// Simple, grouped and external products
add_filter('woocommerce_product_get_price', array( $this, 'custom_price' ), 99, 2 );
add_filter('woocommerce_product_get_regular_price', array( $this, 'custom_price' ), 99, 2 );
// Variations 
add_filter('woocommerce_product_variation_get_regular_price', array( $this, 'custom_price' ), 99, 2 );
add_filter('woocommerce_product_variation_get_price', array( $this, 'custom_price' ), 99, 2 );

// Variable (price range)
add_filter('woocommerce_variation_prices_price', array( $this, 'custom_variable_price' ), 99, 3 );
add_filter('woocommerce_variation_prices_regular_price', array( $this, 'custom_variable_price' ), 99, 3 );

// Handling price caching (see explanations at the end)
add_filter( 'woocommerce_get_variation_prices_hash', array( $this, 'add_price_multiplier_to_variation_prices_hash' ), 99, 3 );


## This goes outside the constructor ##

// Utility function to change the prices with a multiplier (number)
public function get_price_multiplier() {
    return 2; // x2 for testing
}

public function custom_price( $price, $product ) {
    return (float) $price * get_price_multiplier();
}

public function custom_variable_price( $price, $variation, $product ) {
    return (float) $price * get_price_multiplier();
}

public function add_price_multiplier_to_variation_prices_hash( $price_hash, $product, $for_display ) {
    $price_hash[] = get_price_multiplier();
    return $price_hash;
}

The code tested and perfectly works (only) in WooCommerce 3+.

2) For theme version: functions.php file on active child theme (or active theme):

// Utility function to change the prices with a multiplier (number)
function get_price_multiplier() {
    return 2; // x2 for testing
}

// Simple, grouped and external products
add_filter('woocommerce_product_get_price', 'custom_price', 99, 2 );
add_filter('woocommerce_product_get_regular_price', 'custom_price', 99, 2 );
// Variations
add_filter('woocommerce_product_variation_get_regular_price', 'custom_price', 99, 2 );
add_filter('woocommerce_product_variation_get_price', 'custom_price', 99, 2 );
function custom_price( $price, $product ) {
    return (float) $price * get_price_multiplier();
}

// Variable (price range)
add_filter('woocommerce_variation_prices_price', 'custom_variable_price', 99, 3 );
add_filter('woocommerce_variation_prices_regular_price', 'custom_variable_price', 99, 3 );
function custom_variable_price( $price, $variation, $product ) {
    // Delete product cached price  (if needed)
    // wc_delete_product_transients($variation->get_id());

    return (float) $price * get_price_multiplier();
}

// Handling price caching (see explanations at the end)
add_filter( 'woocommerce_get_variation_prices_hash', 'add_price_multiplier_to_variation_prices_hash', 99, 3 );
function add_price_multiplier_to_variation_prices_hash( $price_hash, $product, $for_display ) {
    $price_hash[] = get_price_multiplier();
    return $price_hash;
}

Tested and works on woocommerce 3+

For products in sale you have those hooks:

• woocommerce_product_get_sale_price (Simple, grouped and external products)
• woocommerce_variation_prices_sale_price (Variable products (min-max))
• woocommerce_product_variation_get_sale_price (Products variations)

Cached prices and woocommerce 3:

The 3 filters hooks involved in variations cached prices are:

• woocommerce_variation_prices_price
• woocommerce_variation_prices_regular_price
• woocommerce_variation_prices_sale_price

Introduced in Woocommerce 3, woocommerce_get_variation_prices_hash filter hook will allow to refresh variations cached prices in a much more efficient way, without deleting related transients anytime that this hooks are executed.

So performances will stay boosted (Thanks to Matthew Clark that pointed this better way)

See: Caching and dynamic pricing – upcoming changes to the get_variation_prices method

For filtering product prices with a widget (min and max price), use the following hooks:

• woocommerce_price_filter_widget_min_amount that has one argument $price
• woocommerce_price_filter_widget_max_amount that has one argument $price