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.
At Boston Comic Con this past August, it was $80 for a signature and $80 for a picture. The signature is good for only one item, but the picture however you could have up to four people in it. So you could split the cost of the picture four ways, bringing it down to $20 a person, which is fairly reasonable.