Starting from the premise that you have more miles than you plan to use, and that you are looking to purchase the desired cabin (i.e. no upgrades)—

If you buy it with the card, you can redeem miles for a credit at a rate of 1 mile per cent. So for a $100 ticket purchase, it would take 10000 RDM to fully offset the purchase.

If you look at award pricing, it may be better than 1 cent per mile relative to the price for a cash ticket. So compare the two rates to pick the better one.