Try something like this:
Code:
{if $cart.discounted_subtotal gt 300.00}
<div style="font-weight: bold; color:#00AEAF;">Congratulations, you qualify for Free Shipping!</div>
{else}
<div style="font-weight: bold; color:#00AEAF;">Spend an additional {math equation="x - y" x=300.00 y=$cart.discounted_subtotal} to qualify for Free Shipping</div>
{/if}