The Penis Envy magic mushroom strain (P.E.) is colloquially known as the most potent Psilocybe cubensis shroom variety. It's theorized that this mushroom strain was first isolated by legendary entheomycologist Terrence Mckenna, and extensively isolated over several years to create a significantly higher potency genetic.
 
It effects include vivid visual stimulation, intense feelings of euphoria, and deep introspection/philosophical thought.
 
Its charasteristics consist of large dense stalks with an underdeveloped pale cap, somewhat resembling a phallic object. PE mushrooms drop very few spores and therefore are difficult to propagate/cultivate. This, along with increased potency makes them a rare and desirable strain.
Another notable distinction that separates Penis Envy from almost all other cubensis varieties is the lack of a velum (veil) separating the cap and stem of the fruiting body.
 
It's higher price comes from it's potency and difficulty to grow.  They grow longer with smaller yields.