Wand core

The core of a wand is a magical substance placed within inside the length of wood. The materials used for wand cores can vary widely, though certain wand-makers may prefer to use certain materials; for example, Garrick Ollivander discovered and pioneered the use of phoenix feathers, dragon heartstrings, and unicorn tail hairs, wheras his father used lesser substances such as Kelpie hair.

Price
It is worth noting that Harry's phoenix feather wand cost 7 galleons. Horace Slughorn, however stated that unicorn tail hair was worth 10 galleons a hair. It is possible that wands made with unicorn hairs are more expensive than those made with phoenix feathers. Of course, it is equally possible that in the five years between the time Harry's wand was purchased and the night Slughorn made the comment that the market for wand material had fluctuated greatly (Most likely due to the capture of Ollivander by the Death Eaters, in no small part). It's also possible that wands are subsidized by the Ministry of Magic for students, and that the actual cost of Harry's wand was greater than the 7 galleons he paid. It is also possible that Ollivander collects wand cores himself, as he was nearly gored by the unicorn that gave the core of Cedric Diggory's wand.

Behind the scenes

 * The Enchanted Feather Company is a company that presumably sells wand cores to wandmakers.
 * Peacock feathers may possibly be used as wand cores.
 * It is thought by some that Thestral hair makes for a unique personality; completely dispassionate about its allegiance and showing no emotion towards the wizard, tying in with the many changes of hands that the Elder Wand (the only known Thestral hair wand in existence) has been through. It is also a wand that has been the instrument of death many times over, which some also believe is connected to its Thestral hair core (possibly inspiring murder in the heart of its beholder(s), since Thestrals have a strong connection to death, as does the origin of the wand itself). However, the many changes of allegiance that the Elder wand has had as well as its history of winning through murder may be simply a reflection on the nature of power itself, and the means people are willing to sink to in order to attain it.
 * It should be noted that, concerning the wands of the Trio (Harry, Ron and Hermione), each wand has one of the three different cores used by Ollivander: Harry's wand has a phoenix feather, Ron's wands (both his first and second) have a unicorn hair and Hermione's wands (both hers and Bellatrix's) have a dragon heartstring.

Appearances

 * Harry Potter and the Deathly Hallows 
 * Harry Potter and the Deathly Hallows: Part 1
 * Harry Potter and the Deathly Hallows: Part 2
 * The Wizarding World of Harry Potter