Set price to 0 for a perticular product.

functions.php

<?php

function add_custom_price( $cart_object ) {
	$cart_total 		  = 25; // If cart total is more than this value.
	$free_product_id 	= 6332; // Set price to 0 of this free product.

	$carttotal = 0;

	foreach ( $cart_object->cart_contents as $key => $value ) {
		$_product = $value['data'];
		if ( $_product->get_id() == $free_product_id ){
			continue;
		}
		$carttotal += $value['line_total'];
	}

	if ( $carttotal >= $cart_total ) {
		$custom_price = 0; // This will be your custome price  
	    foreach ( $cart_object->cart_contents as $key => $value ) {
	    	$_product = $value['data'];
	        if ( $_product->get_id() == $free_product_id ){
	        	$value['data']->set_price( $custom_price );
	        }	        
	    }
	}   
}
add_action( 'woocommerce_before_calculate_totals', 'add_custom_price' );
?>