Get product's default variation ID, variation price or variation object without plugin.

function.php

function tv_find_matching_product_variation( $product, $attributes )
{ 
    foreach( $attributes as $key => $value )
    {
        if( strpos( $key, 'attribute_' ) === 0 )
        {
            continue;
        }
        unset( $attributes[ $key ] );
        $attributes[ sprintf( 'attribute_%s', $key ) ] = $value;
    }
    if( class_exists('WC_Data_Store') )
    {
        $data_store = WC_Data_Store::load( 'product' );
        return $data_store->find_matching_product_variation( $product, $attributes );
    }
    else
    {
        return $product->get_matching_variation( $attributes );
    } 
}

//on product single page.
/* $attributes something like this. */
/*$attributes = array(
    'pa_size' => 'small',
    'pa_color' => 'red',
    'pa_brand' => 'nike'
);*/

$variation_id = milkbed_find_matching_product_variation($product, $product->default_attributes);
$variable_product = wc_get_product($variation_id);
//$default_variation_regular_price = $variable_product->regular_price;
//$default_variation_sale_price = $variable_product->sale_price;
$default_variation_price = $variable_product->price;