Change the purchase button text and append the price in Easy Digital Downloads

gistfile1.php

<?php

function edd_custom_edd_checkout_button_purchase() { 

	global $edd_options;

	$color = isset( $edd_options[ 'checkout_color' ] ) ? $edd_options[ 'checkout_color' ] : 'gray';
	$color = ( $color == 'inherit' ) ? '' : $color;
	$style = isset( $edd_options[ 'button_style' ] ) ? $edd_options[ 'button_style' ] : 'button';

	$button_label = 'Pay ' . edd_currency_filter( edd_format_amount( edd_get_cart_total() ) );
	ob_start(); 

	?>
	<input type="submit" class="edd-submit <?php echo $color; ?> <?php echo $style; ?>" id="edd-purchase-button" name="edd-purchase" value="<?php echo $button_label; ?>"/>
<?php 
	return ob_get_clean();
}
add_filter( 'edd_checkout_button_purchase', 'edd_custom_edd_checkout_button_purchase' );