Keeping in line with detailing ideas posted by Xavi here is my attempt at the following spell.
The base is for a fire doing +5 damage, which is the lowest level of fire that can be conjured with Creo Ignem. This is base 4.
The flaming circle
Level 15 CrIg
Range: Touch, Duration: Ring, Target: Individual
base 4 +1 touch +2 circle.
This spell creates a flame that burns within the circle for a long as the circle remains undisturbed. This flame can be used to heat rooms, cook food, and provide light.
How much is this worth in savings? This spell is sort of borderline when it comes to casting it as non-fatiguing spontaneous magic. The savings from a single casting is also debatable. Really the benefit here is that its duration is potentially infinite and replacing all of the fires in the fireplaces in the covenant can be done by a group of magi using fatiguing spontaneous magic over a period of about a season. If used to replace most of the fires in the covenant it can save considerable sums of money by making fuel and candles unnecessary and providing heat for cooking. This can save labor if the covenant relies of labor to gather fuel or money if the covenant buys fuel. I personally think it is pretty fair comparison to the three laborers worth of savings provided by the woodshed of plenty in Through the Aegis p. 152. It could also be treated as a magnitude 2 effect that provides savings on consumables meaning that it saves 2 pounds worth of consumables.
Edit: I mistakenly wrote Duration:Circle, instead of Duration: Rind and I initially miscalculated the level to 10th and I have adjusted the above so that the spell in now correct.