If you mean by price possibly, if you mean ingame there is a lifespan for flames to appear for certain time, like if its set to a 30 turnframe youd see it only every 30 frames.
Other than that no they don't. Unless they get trapped on an account that went inactive or if it goes to an item dump.
Well some flames have become invisible. At some point they changed somthing at how they work and many have become invisible. So yes, some flames die, or have died at some point. The item still exists but it dosen't show up in game.
If there will be more modifications in the future it's very possible that more will die