United is running an offer to give bonus miles on buying miles, here is the mailer Screenshot -
Its a tiered offer,
no bonus miles till 4k points,
5k to 19k - 50% additional bonus miles
20k to 39k points - 70% bonus miles
40k to 89.5k points - 95% bonus miles
Buying miles is usually a bad value, and this is still the case specially with the recent devaluation from United. Even at 100% it's still a bad value unless you have a very specific redemption in mind.