Hmm I cant really define why it's the expensive stuff but the site I bought it from had a few options and the clay I got was 5 times as expensive as wed clay. As it was a birthday present and my first go at sculpting I thought spare no expense
The advantage I found of this over bondo is no sanding (ofc), no toxins so can be done in the house and if a bit isn't quite right its easy to changed it. The only down side is that due to it never drying you have to be really careful not to mark the clay but if you do its easy enough to put it right.