There were some good guidelines in Tome&Blood.
Even if it is 3.0, it might be a good starting point, when paired with the guidelines from the DMG (like the level/damage (1 target)/AoE - caps).
The next step would be to compare the new spell with existing spells. Benchmark lists may help narrow down the range. Posting the spell and asking for evaluation is the next step - and playtesting the final one.
Let's see, benchmark spells (partially based upon T&B, partially upon my opinion).
Level 1
Offensive spells:
Magic Missile, Charm Person, Sleep
Defensive: Mage Armor, Shield
Miscellaneous: Feather Fall, Magic Weapon
Level 2
Offensive: Glitterdust, Acid Arrow, Scorching Ray
Defensive: Blur, Protection from Arrows
Misc:
Invisibility, Knock, See Invisibility
Level 3
Offensive: Fireball, Stinking Cloud, Suggestion
Defensive: Blink, Displacement
Misc: Fly, Haste, Dispel Magic
Level 4
Offensive: Charm Monster, Enervation, Phantasmal Killer
Defensive: Stoneskin, Minor Globe of Invulnerability
Misc: Dim. Anchor, Dim. Door, Imp. Invisibility,
Polymorph
Level 5
Offensive: Cloudkill, Cone of Cold, Dominate Person
Defensive: Wall of Force / Iron
Misc: Teleport, Dismissal, Mind Fog, Passwall, Permanency
Level 6
Offensive: Chain Lightning, Disintegrate, Mass Suggestion
Defensive: Antimagic Field, Globe of Invulnerability
Misc: Greater Dispel Magic, True Seeing, Mass [Ability Booster]
Level 7
Offensive: Finger of Death, Delayed Blast Fireball, Prismatic Spray
Defensive: Spell Turning
Misc: Phase Door, Forcecage, Ethereal Jaunt, Limited Wish
Level 8
Offensive: Horrid Wilting, Mass Charm, Power Word:Blind, Trap the Soul
Defensive: Mind Blank, Protection from Spells, Prismatic Wall
Misc: Dim. Lock, Polymorph Any Object
Level 9
Offensive: Dominate Monster, Energy Drain, Wail of the Banshee,
Lower limit for damage spells |
(Meteor Storm)
Defensive: Prismatic Sphere, Time Stop
Misc: Gate, Etherealness,
Wish