Pricing of burn spells is odd. You can get 3 damage to any target for instant with an upside (Incinerate), but 4 damage to any target costs instant (Lightning Blast; compare Flame Javelin). Downgrading to sorcery speed gets you a minor upside like a conditional cantrip.

Yeah, I know what you mean, although I've not done anywhere near as much limited developing as you. It's definitely on multiple axes. It seems there was a wide range of acceptable costs, and wizards deliberately print one or two burn spells per block that are are at the high end, so (a) they don't make a critical mass in standard/extended and (b) they can slowly fill in the possibilities over time. Eg. You could probably print "RR. lightning bolt for 4", I think, but it would be approx as good as lightning bolt, so they'll save it till one day they want to push red burn spells and make a splashy preview.

So cost is something like [base cost based on damage dealt] + [cost for being an instant] + [cost/discount for benefit/drawback/restriction] + [extra cost if it can hit creatures for three or four at instant] + [extra cost if it can hit players for four or five ever] + [cost handicap if there's another really good burn spell in the block] + [random variance so some are better than others]...

Pulled up to uncommon because there's a lot of damage-dealing at common, and no blocker-prevention. New Aerial Chaos will stop some creatures blocking.

Down to common again, swapping with Skysparker.