Transact SQL :: How To Turn Select Aggregate Function Statement Into A Variable

I tend to learn from example and am used to powershell. If for instance in powershell I wanted to get-something and store it in a variable I could, then use it again in the same code. In this example of a table order items where there are order_num, quantity and item_prices how could I declare ordertotal as a variable then instead of repeating it again at "having sum", instead use the variable in its place?

Any example of such a use of a variable that still lets me select the order_num, ordertotal and group them etc? I hope to simply replace in the "having section" the agg function with "ordertotal" which bombs out.

select order_num, sum(quantity*item_price) as ordertotal
from orderitems
group by order_num
having sum(quantity*item_price) >=50
order by ordertotal;