Through Tuesday (December 6, 2016), you can buy IHG Rewards Club points with a 100% bonus.

Until December 5, 2016, you can buy 10,000-160,000 Hilton HHonors points for 50% off.

Hilton points normally cost 1 cent each. With the 50% discount, the price drops to 0.5 cents per point. This is the cheapest I’ve ever seen them, and is the exact same price as Hilton's more common "100% bonus" sales.

Here is the Hilton award chart.

When to Buy (Math)
Is buying Hilton points for 0.5 cents each a good deal?

I would not buy Hilton points speculatively for 0.5 cents each because I value them at 0.4 cents each.

Through November 16, 2016, you can buy IHG Rewards Club points with a 100% bonus.

Until Sunday (October 30, 2016,) you can buy Club Carlson points and get a 75% bonus on the points purchased. (I've been getting an error after signing in through that link. On the error page, add an "o" to the end of the URL, and it should take you to the correct page.)

Club Carlson points normally cost 0.7 cent each.

Until October 31, 2016, you can buy between 10,000 and 55,000 Hyatt Gold Passport Points and get a 40% bonus on the number of points you purchase.

Hyatt points normally cost 2.4 cents each.

Until October 21, 2016, you can buy Hilton HHonors points and get a 100% bonus on the points purchased.

Hilton points normally cost 1 cent each. With the 100% bonus, the price drops to 0.5 cents per point. This is the cheapest I've ever seen them.

The bonus only kicks in on purchases of 20,000 (+ 20,000 bonus.) Purchases of 10,000 to 19,000 points have an 80% bonus, bringing the price to 0.555 cents per point.

Through Friday (September 30, 2016), you can buy IHG Rewards Club points with a 100% bonus.

I just wrote about United's mile sale (lasts through August 15, 2016) where you can get up to 100% bonus miles.

Through August 15, 2016 at 11:59 CT, United is offering a 100% bonus on purchased miles. The size of the bonus depends on the number of miles you buy, with the largest 100% bonus kicking in at 40,000 miles purchased.

United miles normally cost 3.5 cents per mile plus a 7.5% tax, bringing the full price to 3.76 cents each.

Until July 5, 2016, you can transfer 20,000 Starpoints to 33,750 British Airways Avios. The normal rate is 20,000 points to 25,000 Avios, but Avios is running a 35% bonus on transferring hotel points.

The only hotel-point transfer option with a decent rate is Starpoints.

If you don't have Starpoints, you can buy them right now for 2.45 cents each. The best number to buy would be 20,000 for $490 to maximize this promotion, since Starwood gives a bonus for every 20,000 Starpoints transferred to airlines.