At which point, I fall back to where I've been before, of wondering why we can't tie extended rests away from solar cycle (sun went up and down, whee more spells for me!) to something on the DM side. Like make spells recharge when the PCs level (with guidance for making that every N XP, or whatever, instead) or when the DM decides otherwise (Ex: 'You've defeated the Lich King! You're still shy of leveling, but you do recover all of your abilities during the celebrations that follow.')
Part of it is because logically where this kind of thinking carries us, when we look at the mechanics and handling time, is a place a lot of people don't want to go. (Kind of like how making Armor as AC only makes sense if you work it through, only in reverse.)
To wit, consider having two forms of resting. We'll call the natural form "resting" and the more magical form "recharging" for sake of example. Then you might have something like this:
Short Rest - lets you bandage some wounds, drink potions and cast spells freely, etc. All it really means that if you aren't pressed right this minute, and have got some charges left in the wand of cure light wounds
, you can cast 'em if you've got 'em.
Short Recharge - takes about the same amount of time as a short rest and can thus overlap it (i.e. individual party members can do one or the other). You regain some hit points and/or some "encounter" abilities. In Next, you can use a hit die.
Long Rest - get back hit points equal to your level, plus Con Mod. Regain a limited
number of Vancian spells. Begin the long process of shrugging off long-term effects, such as 3E ability damage healing. You can do this once per day.
Long Recharge - Boom! Get back all your hit points and spells, or very close anyway. Regain ability damage and other long-term effects in a major way, if not fully. Overlaps with Long Rest the same way the short variants overlap with each other.
Now, to bring that back to what you were proposing, charge XP for the recharge effect
, and base the cost on the level of the character getting the recharge. I'd start with something around 3% to 5% of what is needed for a level, but have this gradually increase over the course of an adventure, perhaps modified by events. You might even include a "coward" penalty if the party novas against very weak opponents and then rests.
It works out about the same as adjusting the final XP based on expected play, but is more organic in that the players are getting immediate feedback. Plus, they can very clearly set their own tolerances. That is, if one group hates to lose any XP, they can work hard not to do so. Whereas another group might consider throwing in a rest or two every adventure part of the price of doing "efficient" business.
Of course, as I've said elsewhere, this would work even better if tied to a robust and fluid action point currency or something similar. If you can use action points for extra actions, but also use them to "recharge" abilities, it becomes a very serious trade between rapid, less effective options versus more measured, more effective options.