United has launched a 4 day mile sale within Mileage Plus. The deal is pretty good if you need to top off your accounts and you missed the last sale a few weeks ago.
Like with other sales, the more miles you buy, the better the bonus:
Buy 5,000 miles or more, get a 20% bonus (5,000 + 1,000 = 6,000 miles for $175.00)
Buy 15,000 miles or more, get a 40% bonus (15,000 + 6,000 = 21,000 miles for $525.00)
Buy 30,000 miles or more, get a 67% bonus (30,000 + 20,100 = 50,100 miles for $1050.00)
Keep in mind that you are limited to purchasing 150,000 miles per account per year.