Upgraded Fire Spectres cost more.
Fire spectres don't cost more than fire spirits except for anti-decking and immolation reasons (which are enough here I guess, if you're worried about losing pillars and the game going on long enough for you to deck out) and are usually a lot stronger...
Either way you are spending 3 fire quanta for a 2 attack creature, 3 for a 4 attack creature, 4 for a 6 attack creature, etc. The only difference is that spectres are always a step ahead.
Since you have to wait until at least turn 4 to play them (Anubis will almost never come into play until turn 3 or later, makes the fire spirit immortal on turn 4 at best, or 5 if it does itself first), and since you have more fire pillars than fire spirits, it's safe to say you usually won't be able to play the fire spirit a turn earlier than a spectre--the only situation where this happens is when you draw your first fire pillar after you can play your anubis and have a fire spirit in your hand.
If you can play a spectre the same turn as a spirit, it does 2 more damage every round... that really adds up.
If you can play the spirit one turn earlier than you would have played the spectre, they do equal damage: 2 on the second turn, 4 on the third, etc.