Fire crystals sell for between 1800 and 2400 depending on time of day, supply, and demand at any given time.
Dark crystals sell for between 3000 and 3600 depending on the same, but they do sell a little slower.
Light crystals sell for between 6700 and 7500 depending on the same, but they sell slower than Dark Crystals do (there's not a lot of demand).
Earth crystals rarely sell for more than 300 gil.
Water, Wind, and Ice crystals sell for around 1400 gil.
Lightning crystals sell for between 1800 and 3000, I don't know why the price skitters around so much.
All referenced prices are related to two things: a Stack of 12, and prices on the Garuda server. I don't imagine they'd be insanely different on other servers.