Flaming Missiles is described on M75.
Its Enchantment version mentions "at no cost to the user" but is unclear whether that means after spending the usualy energy cost to turn it on for minutes.
The cost of 1,000 energy is already the cost for Power 2 which would be required to maintain that indefinitely, and even then it would cost 2 to turn it on.
Presumably the 1,000 would also double to 2,000 for stone/metal missiles.
Magic Items 1 is no help as the item has the note "Allows the user to cast the spell exactly as if he knew it himself" which contradicts the "at no cost to the user" part.