Core fibre is the real work horse of needle felting! It's texture makes it quick and easy to felt making it perfect for using to create foundation shapes for your projects.  Its low price means that you can save your pennies building up with this, money that you can invest in better quality wools to felt over the top. When you're done all anyone will see is the more luxurious wool fibres! I definitely prefer making my own shapes with this rather than using the Styrofoam alternatives that don't always hold together well under a good stabbing. 
A 50g bag goes a long way and will definitely expand once opened so be prepared to fight it back into its resealable bag once done!