The price of a wand is determined by its magical ingridient (such as a phoenix feather) and the type of wood. In the order form, where it says to describe your wand, you could write something like this: Willow, 9 and a half inches, Unicorn hair. Or Pine, 11 inches, Dragon Heartstring.
You determine the type of wood by your sign, if you are a Scorpio like myself, you will need to find a type of wood that is very strong like Red Oak. Or if you are and Aquarius or Pisces, you'll need to find a pretty, graceful and modest tree, that needs a lot of water. You only need to know a few characteristics of your sign and then choose a tree that has some or most of those qualities.
The size, really depends on how big and how strong you are (you wouldn't want a wand that is too heavy or too light for you) Like Harry, his wand is 11 in., Voldemort's is 13 in. and Hagrid's used to be 16 in. That is really up to you.
The magical ingredient is absolutely up to you, of course some are more expensive than others. If you buy the rarest just because it's supposed to be more powerful, it won't work, you need to put the magical ingredient that you feel goes most with your personality, a wand with 5 phoenix feathers in the hands of someone who is not right for it, won't even be able to do the most basic spells.
Basilik scales
Giant's toenails
Hippogriff beak
Poisonous Tarantula legs
Dragon Heartstrings
