If your front yard if flat you need 21 stones for each layer your would like to build. A typical retaining wall around a tree would require 3 layers. If your front yard has a grading, you have to see how many stones you need depending of the grading.
How much does it cost? As of 2010, the 4-day passes to Comic Con were $100 each. However, it increased as the years went by. According to WhenNerdsAttack.com, the 4-day pass with preview night costs $175 for adults and $87 for juniors, military and seniors.
Like any other events, tickets for Comic Con are a lot cheaper if they are purchased months before the event. According to Mountaincon.org, the earlier one registers, the cheaper the ticket will be. For example, one can register for 2013 at the 2012 Comic Con.