Default Money Formatting for the US

functions.php

<?php

function set_us_dollars_localeconv( $locale = array() ) {
	$locale = array(
		'decimal_point' => '.',
		'thousands_sep' => '',
		'int_curr_symbol' => 'USD',
		'currency_symbol' => '$',
		'mon_decimal_point' => '.',
		'mon_thousands_sep' =>  ',',
		'positive_sign' => '',
		'negative_sign' => '-',
		'int_frac_digits' => 2,
		'frac_digits' => 2,
		'p_cs_precedes' => 1,
		'p_sep_by_space' => 0,
		'n_cs_precedes' => 1,
		'n_sep_by_space' => 0,
		'p_sign_posn' => 1,
		'n_sign_posn' => 1,
		'grouping' => array(),
		'mon_grouping' => array( 3, 3 ),
	);
	return $locale;
};
add_filter( 'si_localeconv', 'set_us_dollars_localeconv' );