This will do the trick. The negative length in the substr call will trim the end for you.
<?php
$myPrice = $_coreHelper->formatPrice($_price, false);
$myPrice = substr($myPrice, 0, -2);
$myPrice = $myPrice . '-?';
echo '<span class="price">'.$myPrice.'</span>';
?>
result : €1.100,-?
Echoing the <span>
tags is up to you, you don't necessarily need the container but you may want to have it, or to set another class, etc.
To filter by prices with '00' as the decimals, and only insert the symbols on those prices:
<?php
$myPrice = $_coreHelper->formatPrice($_price, false);
$zeros = substr($myPrice, -2);
if(strval($zeros) == "00") {
$myPrice = substr($myPrice, 0, -2);
$myPrice = $myPrice . '-?';
}
echo '<span class="price">'.$myPrice.'</span>';
?>