I've been looking at the Wish spell the other day. In Castles & Crusades, Wish will cost the spellcaster 1 to 3 years of life. Yeah, I'm sure that's a huuuuuge setback to the ELF spellcaster in the group! (In the C&C Player's Handbook, it says an Elf becomes Middle-Aged at 500 years-old!)
Even if I did some math to find the equivalent for an elf in years, it still leaves the player with a character who would live for a ridiculously long time and still keep going. An elf in Castles & Crusades could just cast it daily and become the strongest, wisest, most dexterous and charming character of all time. Unless compared to other magic-using elves, I guess.
Oh sure, if you bring it up in some message boards people will start telling you how you could potentially screw over the players when they use the spell. Unless a player really does word his Wish poorly, I'm not really interested in going there.
Now, as far as spellcasting goes I mostly use the rules from Lamentations of the Flame Princess. One thing I don't use from LotFP and go with the C&C ruling instead is daily spell preparation time. I'm really not interested in a Magic-User needing one hour per spell level to prepare all his spells. There's such a thing as prep time and downtime for characters, but I think that's taking it a bit too far. 15 minutes per spell is just fine.
One interesting aspect of LotFP Magic-Users that I don't think other OSR games have done is that spellcasters can cast and even write spells that they wouldn't normally be able to cast at their current level. They cannot prepare them as one of their normal daily spells, but they can read them from scrolls. So in theory, a Level 1 character could actually write down a Level 9 spell in his spellbook and then write it on a scroll to use it later on!
I can imagine many who are unfamiliar with the system just gasping in horror at the thought. The potential for abuse! Actually, not so! Sure, in theory they can do it... But it's very prohibitive! (Unless you give away too much treasure, and those who've read my blog recently know I have the opposite problem!)
Imagine a Magic-User finds a Level 9 spell on a scroll. If they cast Read Magic they'll be able to use it, but the powerful spell will be gone from the scroll! Therefore it's much more likely that the Magic-User will write it down in his spellbook first. That will take <Spell Level x 1d6> days, at a cost of 20 GP per day. Except the Magic-User has to say how much time he'll spend doing so BEFORE the roll. If it fails, all the money was spent for nothing and he has a 10% chance to be cursed! Oh and that's actually if the character has a laboratory worth at least 1000 GP x the Spell Level, otherwise the time it takes is DOUBLED. Many characters, unless they are in a desperate rush, will probably choose to just take the maximum amount of days to write down the spell, which will cost a fortune.
Of course, it's not over. Since our Magic-User is too low-level to cast a Level 9 spell, he has to write it down on a scroll next if he wants to use it! 50 GP per day and it takes <Spell Level x 2d6> days. Once again that time is doubled if you don't have a lab!
So right there, Wish has already become prohibitive enough. Unless of course you get to the point where a character can actually cast it as one of his daily spells. Watch that Elf laugh his ass off as he loses a mere 3 years of his life! (Or whatever that will be in Elf years from the point of view of the very human player.) So as mentioned earlier, that won't do as a balancing factor.
Let's look at some books adored by OSR fans; Dungeons & Dragons 3.5 and Pathfinder.
D&D 3.5 has 5000 XP as the cost of the spell. I could calculate approximately what the C&C/OSR equivalent in XP that is, but let's look at Pathfinder first. The spell has a cost of 25 000 GP. Yeah, let's go with that.
Ever since I realized I'd been too stingy with treasure for my players in my OSR campaign where treasure is the main way to level up, I've also read a lot about how rich characters can become down the line and what a problem that can become. Bill Webb's Book of Dirty Tricks even has many of those dirty tricks there just to deal with that problem. It seems to me that it wouldn't hurt to have as many things as possible cost lots of gold then, it keeps the characters hungry for more and adventuring into dungeons!