<?php

$template_text = "{{some template
|example=$123.45
}}";

$existing_page_content = "
Foobar

== Example ==
{{{insertionpoint}}}

";

$existing_page_content = preg_replace( '/\{\{\{insertionpoint\}\}\}(\r?\n?)/',
                                                preg_replace( '/\}\}/m', '}�',
                                                        preg_replace( '/\{\{/m', '�{', $template_text ) ) .
                                                "{{{insertionpoint}}}",
                                                $existing_page_content );

echo $existing_page_content;

Foobar

== Example ==
�{some template
|example=3.45
}�{{{insertionpoint}}}