Endur said:
D&D magic is not unbalanced.
BUT it takes a careful GM to make sure its not unbalanced. If you don't let the magic users get away with breaking the rules, you should be ok.
Something else to remember... characters don't always spend money on magic items.
High level Wizards have to buy spell books and research spells. This costs tons of money. High level Wizards will rarely have any magic items other than their spellbooks.
High level Clerics need to establish a Temple. Again this costs lots of money.
High level Fighters can build a fortress again, costing lots of money.
So, your high level PCs/NPCs, even if they have a high GP value, might not have any magic items, because they might have spent their loot on more valuable items than magic.
Actually, clerics and fighters don't need to do either of those.
Wizards without cloaks of resistance are asking for death by some Fort spell. And besides, spells and research aren't nearly as required as you make out.
I don't "need" to research spells as a high-level wizard that I can negotiate with a guild or other wizard to trade with me for. If I wanted a better version of a PHB spell, and the DM has not already approved one as in widespread use, sure I'll research it. But I could just decide that researching is too much trouble.
Besides, we aren't talking about scrolls here either. IMO, I would buy scrolls before I researched a spell.
I'm on "your" side, but you aren't helping "us."
Besides, the mention of Contact Other Plane as some sort of fighter-killing spell (it's probably more dangerous to the wizard casting it) makes me think that your concerns are mostly theoretical, or maybe based on one incident where the circumstances went against the fighter.
This has to be the worst example of wizard power ever. There are no guidelines for what these powers know, even in 3.5. IMO, I have no idea how one would use information that they would know to beat a fighter. So it's really up to the DM.
IMO, you could ask where a center of power for that specific being contacted, or if there was a cleric nearby of that deity that would cast a powerful spell for the party or some question about some aspect of the power contacted. How this would directly help beat a fighter, I don't know...