There are many different sized and colored Rune Cubes, each with their own NPC sale price.
Rune Cubes come in an assortment of different sizes and colors. Most enemies, if not all, drop Rune Cubes. Higher level enemies drop bigger Rune Cubes, just as bigger cubes sell for greater amounts of money.
The color of the cube varies with its price, being in this order of most valuable to least:
White > Green > Red > Blue > Yellow
The Yellow cube is the cheapest of all the colored cubes of the same size.
|14 G||11 G||8 G||5 G||2 G|
|38 G||33 G||28 G||23 G||18 G|
|85 G||76 G||66 G||57 G||47 G|
|237 G||??? G||??? G||114 G||??? G|
|??? G||332 G||??? G||??? G||190 G|
Images from JP Concerto Gate wikiki.