Iberia Plus is currently offering a 20% bonus on purchases of Avios points. This deal runs until 27th January, see here for further details. Note that you need to log-in to see details of the bonus offer.
At the top end, you will be paying Euro 459 for 28,800 Avios (24,000 + 20%). That works out at 1.32p per Avios, based on £381. You would need to add a 3% foreign exchange fee to this price if your credit card has one.
This is cheaper than buying Avios via BAEC or avios.com, where you will pay £399 for 24,000 (1.66p). However, you must have been an Iberia Plus member for 90 days before you can transfer Avios across to BA.