United Airlines is currently offering a 20% discount on buying miles–which is additive with the current tax hiatus (yes, airlines did raise their rates to cover the tax break, but that doesn’t affect buying miles). This is not quite as good as it sounds, however. The cost of their miles has also increased to $35/1,000 miles and is then discounted to $28/1,000 miles until 31 July. To somewhat make up for that, there is no longer a “processing fee” to purchase miles. You can tack on the tax break and the savings go up to just about 25% from the new $35 cost. I don’t know what the old cost was (if a commenter fills me in, I’ll edit the post to outline the actual savings).
I’m not big into buying miles unless it’s just to top off an account for an award that is about to be booked since you can usually get miles for free if you try. However, if you’re in that latter category, and UA is your airline, then this represents a good deal. The minimum purchase is 2,000 miles for $56.