Change "$" to "US$" for the PMPro US Dollar currency symbol.

pmpro-currencies-add-us-before-dollar-sign.php

<?php
/**
 * Add "US" as a prefix to "$" for the US Dollar currency code ("US$").
 *
 * You can add this recipe to your site by creating a custom plugin
 * or using the Code Snippets plugin available for free in the WordPress repository.
 * Read this companion article for step-by-step directions on either method.
 * https://www.paidmembershipspro.com/create-a-plugin-for-pmpro-customizations/
 */

/**
 * This function changes the values of specific keys for a currency in the `$pmpro_currencies` array.
 *
 * @link https://github.com/strangerstudios/paid-memberships-pro/blob/dev/includes/currencies.php
 *
 * @param  array $pmpro_currencies The currency array created by PMPro
 * @return array                   The adjusted currency array
 */

function my_pmpro_customize_us_dollar_currency( $pmpro_currencies ) {
	$pmpro_currencies['USD'] = array(
		'name'                => __( 'US Dollars (&#36;)', 'paid-memberships-pro' ),
		'decimals'            => '2',
		'thousands_separator' => ',',
		'decimal_separator'   => '.',
		'symbol'              => 'US&#36;', // e.g. US$25.00
		'position'            => 'left',
	);
	return $pmpro_currencies;
}
add_filter( 'pmpro_currencies', 'my_pmpro_customize_us_dollar_currency' );