Originally Posted by DCBob
Even if you have a refund for 2005, you can make an estimated tax payment for 2006 (whether you need to or not). This can really pay off:
Example: Charge $5,000 and pay a fee of $124.50. Receive 10,000 SPG points, which can buy 1 night at the Sheraton Park Lane in London. Cheapest rate at the Park Lane is around 149 pounds = $260.
Net gain = $135.50, or more than a 50% discount on the room.
Except, assuming you don't need to make an estimated payment, you are losing out on a year's interest on that $5,000. Lots of banks paying 4.25% out there, so the lost interest is 5000*.0425=$212.50. $212.50+$124.50=$337>$260.
(Ignoring taxes, and assuming interest rates stay at 4.25% for the rest of the year.)