Through March 28, 2013, British Airways is offering up to a 25% bonus on the purchase of Avios. If you purchase between 5,000 and 23,000 Avios you earn a 15% bonus, while if you purchase 24,000 Avios (the maximum number that can be purchased per account per year) you earn a 25% bonus.
If you purchase 24,000 Avios you get 6,000 bonus Avios for a total of 30,000 Avios, at a cost of $685. That’s ~2.28 cents per Avios.
That’s not a cost at which I’d purchase Avios just for the sake of stocking up, though there are some gems on the British Airways award chart for shorthaul travel, so in some instances this could prove worthwhile.