Through November 30, 2012, Starwood is offering a 20% discount on the purchase of Starpoints, bringing down the cost to 2.8 cents per point. This is a promotion they seem to run a couple of times per year.
You can purchase a maximum of 20,000 Starpoints per calendar year, so taking advantage of this promotion costs a maximum of $560.
I value Starpoints at roughly 2.2 cents each, but many value them higher. Keep in mind you can transfer Starpoints to many airline programs at a 1:1 ratio, and for every 20,000 points you transfer you get a 5,000 point bonus. So spending $560 would net you 25,000 miles with dozens of airline programs, for a total of ~2.2 cents per mile. That’s not great, but not awful either.
But the real value in Starpoints is the ability to use them for cash & points redemption, which can easily net you way over 2.8 cents per point in value.
This isn’t a promotion I’ll be taking advantage of, but I know of plenty of people that take advantage of the promotion every time Starwood runs it.