Make a [B] thread
here and state what you're looking for.
The pricing of the flame depends entirely on the seller, usually ranging from 25k - 35k for a typical flame. The color doesn't affect the forge price, but in my experience, black/void flames tend to be more expensive (maybe because they can be changed to any color using a FPT).
Technically speaking, the cheapest flame you could get from another user would probably be a broken one with no visible particles.