My players like to use spellstones. Basically, crushable gems containing a spell.
They're handy because they turn just about anyone into a spellcaster - you crush it, you cast it, and it works like any other spell. This makes it really useful for spells you want at hand to aid friends or annoy foes.
The breakpoint for "quick and dirty" casting versus "slow and sure" enchantment in Dungeon Fantasy is at 100 points. Some enchantments get the favorable break and cost 1 x 100 = $100. Others, such as Deflect, get tossed in on the far side of the line and cost $2000. Yes, 100 costs either 100 or 2000, depending.
Energy 1 = 50 (stone) + 20 (spell) = $70
Energy 2 = 120 (stone) + 40 (spell) = $160
Energy 3 = 210 (stone) + 60 (spell) = $270
Energy 4 = 320 (stone) + 80 (spell) = $400
Energy 5 = 450 (stone) + 100 (spell) or 2000 (spell) = $550 (or $2450)
Energy 6 = 600 (stone) + 2400 (spell) = $3000
. . . and so on, using the formula from GURPS Magic p. 61
The question on a 5-energy spell is whether you round up ($2000) or round down ($100). Like the aforementioned Deflect, spells that are especially effective for their casting cost might get the round-up. After all, take a spell like Great Haste, it's extremely valuable. The demand can be potentially really high. And meta-game, the abuse potential of "everyone has Great Haste ready to go at all times" might make a higher cost make sense. For Great Haste at $550 each, it's a great deal. At $2450, would you still pay? Very possibly. 10 seconds of Altered Time Rate at a cost of 5 FP at the end is pretty freaking awesome, and what if you don't have to wait for the wizard to put it on you guys one at a time?
There is no round-up question for energy 1-4, though. That's probably another strong argument for "next game, all enchantments cost $20/point" though, since it means magical gear does have a baseline cost that's higher than the material cost of much of what you put it on.
Note: This is for skill 15. Higher skill is possible, of course, but at least the way I run it is that you pay for higher skill regardless of whether it's from the enchanter using extra power or it's just native skill. Prices are set by a monopolistic guild, not a market economy.
What are some good spellstones?
To purchase, I mean. Anything you can find is worth it, but not everything is cheap enough to make it a worthwhile expendable purchase. Here are three I think are pretty good deals:
Awaken is my PC's favorite spellstone. They buy pretty big ones - 3-4 energy, usually - so people can crush the stone and affect a small area with Awaken. It's very useful for reviving knocked-out allies. Especially key is that you can center it on yourself (-0 to cast), maximizing the margin of success that acts a bonus to revive the unconscious. And unconscious foes? Exclude those hexes, you're a spellcaster for an action, not activating a magic item with pre-programmed area effect.
Invisibility is another interesting choice in DF, since the cost is only 4. $400 for "and then I'm gone" when you're in trouble. Not bad.
Missile Shield is 5 to cast, so it's $550 or $2450. At $550, it's handy to put it up on yourself for a minute without needing to be near your wizard buddy when the arrows start coming in.
Naturally I control these like any other magic item purchase, but they're an interesting option for PCs with cash on hand and a need for quick casting.
And yeah, I'm thinking $2450 for Great Haste in my game. $550 seems a bit too cheap for what you get out of it.
Any spell stones you guys think are a good investment for the cost?