For a short period of time, it could have been obtained via transmutation due to a bug.It was awarded to the Top 100 ranked players during the Information Event in 2013.Primed Chamber could have been acquired in a few ways: The bonuses it grants can be easily matched by well-rolled Riven Mods, but it's considered a legendary collector's item, to which it owes its' astounding price.
Primed Chamber Mod is by far the most valuable and expensive item in Warframe. Price in Platinum: approximately 70,000-100,000 Platinum