Lol I get it they are awesome, it is plastic but you can get all sorts of materials not sure they all work with the pen though - it is a lot like ink but yeh plastic.
The tree itself was over 20 hours continual work, the pen itself was around $100 AUD and the plastic was another $100 ish - I am buying bulk amounts now to lower costs, but the plastic is the true expense here.
Cheers :D