Complex sentence: The island is currently owned by The 8th Marquess of Ailsa but since May 2011, has been up for sale: as of March 2013, the asking price is for offers over £1,500,000, down from the original asking price of £2,500,000.
Split into simpler sentences: The island is owned by The 8th Marquess of Ailsa, but since May 2011 has been up for sale. By March 2013 the asking price was for offers over £1,500,000, down from the original asking price of £2,500,000.