to find the lowest priced item in the cart, you could add a loop through the cart object simialr to:
<?php
$cartName->MoveFirst();
$price = $cartName->DisplayInfo("Price");
while (!$cartName->EOF()) {
if($cartName->DisplayInfo("Price") < $price) $price = $cartName->DisplayInfo("Price");
$cartName->MoveNext();
}
$cartName->MoveFirst();
$_SESSION['50PercentOfLowestPrice'] = $price / 2;
?>
where cartName is the name of your cart object.
This will compare each price in the cart, and if it is less than the previous one, set the $price variable to hold that value. It also creates a session variable that can be used in a flat rate discount.
On the discounts tab, create a new discount, using the trigger Number of unique items is greater then 2.
Use a flat rate calculation of: $_SESSION['50PercentOfLowestPrice']