Went to Crazy Earl, looking at Echo themes, seen Gold as the Grave going for 0 eridium, got it thinking it was a promo. When I went back to look at others, I seen I was down 120 eridium. However there are still 3 other themes listed for 0 eridium; AR-gyle Experience, Titan Up the Graphics and Handheld Hunter. However when another player checked their screen, it was showing the actually price listed.
I had the same issue when playing co-op , I was turning in the mission objective for 0(e) , and it charged me for the echo skin, not sure if the cost was wrong or mission its self is glitch, since leaving and reloading fixed the price but I had no eridium left.