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 only half an Energy Pack, in which case, people will usually buy two Junk Rares for one Energy Pack. Prices do vary however, as some players may buy Junk Rares at a ratio of 3 Junk: 2 EP, while others at a 5 Junk: 4 EP ratio. In some cases, people may value a Junk EX at one EP, due to two junk rares being used to make it. However, others may value it as one card and thus, it would be only half an EP. Though the former is usually the correct price, it can depend on the players preference and value.