I managed to make it work by using $minicart_total_cost. Maybe not ideal, but works.
{if $minicart_total_cost gt 300.00}
<div style="font-weight: bold; color:red;">Congratulations, you qualify for free shipping!</div>
{else}
<div style="font-weight: bold; color:red;">Spend an additional ${math equation="x - y" x=300.00 y=$minicart_total_cost} to qualify for Free Shipping</div>
{/if}