The cost of the energy shown is the Grand Exchange price of one energy. Keep in mind that converting one divine memory requires 5 divine energies and each enriched memory requires 10 divine energy. This is the reason why the cost/xp ratio of using converting divine energy along with memories for extra xp is so high. The experience provided by each energy is always the same regardless of whether you are using them with enriched or plain memories.
It is used to make signs, automatically-activating pocket-slot items with a variety of uses, such as warding off killing blows or protecting extra items on death, or portents, automatically-used inventory items with effects such as healing players who fall below a certain amount of health.
The amount of energies gained when harvesting a spring changes based on level. Energy is harvested from springs regardless of whether the player successfully gets a memory. From levels 1 to 54, the player will only harvest 1 energy at a time. At levels 55 through 74, this is increased to 2 energies per harvest. At levels 75 and beyond, the player will gain 3 energies every harvest.