Display ad in the middle of content on free blogs

Hi,

I am trying to use the following code to display an ad in the middle of post content, and it works really great. But I actually only want the ad to display on our free blogs, and not our paying subscribers (Pro Sites).

<?php

//Insert ads after second paragraph of single post content.

add_filter( 'the_content', 'prefix_insert_post_ads' );

function prefix_insert_post_ads( $content ) {

	$ad_code = '<div>Ads code goes here</div>';

	if ( is_single() && ! is_admin() ) {
		return prefix_insert_after_paragraph( $ad_code, 2, $content );
	}

	return $content;
}

// Parent Function that makes the magic happen

function prefix_insert_after_paragraph( $insertion, $paragraph_id, $content ) {
	$closing_p = '</p>';
	$paragraphs = explode( $closing_p, $content );
	foreach ($paragraphs as $index => $paragraph) {

		if ( trim( $paragraph ) ) {
			$paragraphs[$index] .= $closing_p;
		}

		if ( $paragraph_id == $index + 1 ) {
			$paragraphs[$index] .= $insertion;
		}
	}

	return implode( '', $paragraphs );
}

I usually use the code below to control if ads only are displayed on a certain level, but I don't know if I can't implement it in the above code. I have tried, but it won't work for me.

<?php
			$blog_id = get_current_blog_id();
			$level = 0;

			if ( !is_pro_site($blog_id, $level) ) { ?>

---------- AD CODE HERE ----------

<?php 		}
			else {
				//nothing
			} ?>