Iberia Plus is currently offering a 20% bonus on purchases of Avios points. This deal runs until 13th September, 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 1.26p per Avios, based on £363.37. You would need to add a 3% foreign exchange fee to this price if your credit card has one.
This is far cheaper than buying Avios via BAEC or avios.com, where you will pay £399 for 24,000 (1.66p). However, note that this is exactly the same price (1.26p) that you pay normally when you buy BMI Diamond Club miles, which cost £303 for 24,000.
BMI Diamond Club miles can also be instantly transferred to British Airways Executive Club, whilst you must have been an Iberia Plus member for 90 days before you can transfer Avios across.
There seems little point in taking advantage of the Iberia Plus offer, unless you have already bought your annual maximum of 24,000 via BMI Diamond Club.