Junk Rares, by definition, are Rare cards which have a power of 7 or under, and in most cases with a weak skill (eg. small boost to "X"/ Small drop in "X"). It should be noted definitions do vary among players, and in some cases Rare cards with a power rating of 8 may be considered junk.
Because of their lack of usefulness in combat, Junk Rares are usually worth an Energy Drink or Power Potion in which case, people will usually buy one Junk Rares for one Energy Drink or Power Potion. The recent need for junk rares have increased the price of almost any rare to at least one Power Potion or one Energy Drink. Due to this, people usually value a Junk EX at two ED or two PP, due to two junk rares being used to make it.
In some cases, Junk Rares may be sold at a higher value than normal such as King Dragon or Lunatic Killing Machine. King Dragon, though only a power of 7, has a skill which gives a medium boost to all Defense. Because of the relative usefulness of its skill, it is commonly valued at 1.5 - 2 ED. Lunatic Killing Machine's value is due to its rarity. Though it lacks the same kind of stats and skill as King Dragon, since the card was only given out during Battle Royale II, it exists in very few quantities. Most owners value a Lunatic Killing Machine at 10-15 ED regular, while people with a Perfect Max version will value it at 30 ED.