United is offering a bonus of up to 100% miles when purchasing their miles for a limited time through September 6, 2018. This is a tiered offer and to get the full 100% bonus, you must purchase at least 45,000 miles.
If you go all in and purchase the maximum number of miles (75,000) that will ultimately get you 150,00 miles and will cost $2,821.88. That means you are purchasing miles at 1.88 cents per mile. This is the best promotion you’ll see for purchasing miles and seems to come around once a year.
I personally wouldn’t purchase miles speculatively at this rate, but if you have an upcoming flight you are looking to book it is definitely worth to see if purchasing miles is cheaper than paying the lowest available rate. Also, this could be a good way to top off the number of miles required for an award ticket. You can purchase miles here.
Note: This transaction is from points.com so you will not receive bonus miles if using a credit card that gives a bonus for travel/airline purchases.