Toribash
42k

The flame is really.... well I can't even call it bad. It just isn't there. I'd show you a screenshot, but there wouldn't be anything there. I had to set particle size and emission down to 0 to get it that low.

I'd imagine one could make a decent flame for 50k ish, but as far as "cheapest" goes, this could be it.
Oh, hell, if FNugget could forge a flame only as cheap as 35.7k I'd be inclined to say that 35.7k is the cheapest you can forge them.

/thread