Raising skill levels costs Adventure or Crafting Experience (XP). The amount of XP required is exponential in nature, causing higher-levels to cost considerably more than lower-levels.
The chart below can help visualize the cost of raising skills. Change the Max Skill Level and Factor Cost to get an idea of the XP requirements to get a skill to a particular level.
Along with the base XP cost, each skill also has a Factor Cost. This is a simple multiplier to the calculated XP cost.
Most skills have a Factor Cost of 1. This commonly applies to active skills (e.g., red, yellow, or blue glyphs).
Passive skills (i.e., grey glyphs) quite often have a Factor Cost of 4.
Specializations and wardings (and a couple others) have a Factor Cost of 20.
Check the Adventure and Crafting tabs above for the cost of each individual skill.
The calcuations used on this site differ from other tools I've seen. A lot of other tools use a "lookup" table to calculate the total amount of XP for a particular skill level between 1 and 200. In my testing, some of these appeared to be inaccurate (although, still very useful, as the inaccuracies are relatively small). So, instead, I've chosen to use a function to calculate required XP. I'm convinced this function is also slightly inaccurate, but close enough to be useful.
That XP required function for a skill level is as follows (note that ** is the power operator)...
XP = (xpFactorValue * (Math.ceil(((1.099711**(level-1)) - 1) * 100)))