The cheapest way to do this will to get a National Express coach from London’s Victoria Station to Salisbury. Then once in Salisbury, at the same bus stop you are dropped off at, buy a local bus ticket to stonehenge. The price for this trip will vary depending on what day you are traveling and how you book. I recommend booking your coach from London ahead of time on the internet at http://www.nationalexpress.com They have special internet rates for this route that start at 1 pound each way! The local bus to Stonehenge is just a few pounds. I don’t remember exactly how much, but it was around 2 or 3 pounds.