xp spells gets more "expensive as you level

[you randomly]

spells that cost xp such as planar ally lesser are "cheaper" when you first get them to cast as in when you first get the spell the difficulty of monster you have to kill is easier than when you try and use the spell at later levels as you have to fight more difficult monsters to get the same xp.

for applications like this I think I prefer the system suggested dragon mag 317 "power components"
which suggests extracting parts from the monsters to be used in the casting/crafting process this means that the cost stays the same through out your chars progression

I'm wondering what the people of the forums think of this system
what repercussions this may have on the world
have you ever played with this kind of system or something similar

personally I think this kind of system would work better in a game where the gm tells you when to level

[sleepyphoenixx]

XP gained also raises as you defeat stronger enemies. Since the xp required for Planar Ally scales slower than the xp rewards for higher level encounters you're actually coming out ahead.