They are not particularly expensive: the entry level price for the full kit would be around 100 USD, a negligible amount of money if you are interested in trading P9 and similar stuff
Decks like these are often largely built of cheap fodder creatures, meaning theyre surprisingly affordable for their power level
If Brimaz, Blight of Oreskos is in play, youll incubate as you cast artifact or Phyrexian spells, growing an army of artifact creatures over time
The low supply is another factor in the cards price
As he enters the battlefield, you reveal the top six of your library
With evasion and Deathtouch, Tegwyll can fly into the fray right alongside his court, contributing to a more aggressive gameplan than might initially be expected