No existing item, feat, or class feature allows you to do this. Restoring HP infinitely out-of-combat is an easy trick to accomplish, which turns this into infinite spell slots if you have the time. And it’s pretty easy to make it not take very long at all.
In other words, this is not a balanced idea.
Best case scenario: no one goes out of their way to abuse this
The closest comparison available is a pearl of power; this replaces a 1/day limit with an HP cost. Well, what does it cost to restore HP?
A wand of lesser vigor costs 750 gp, has 50 charges, and regenerates 11 HP per charge. That's 550 HP for 750 gp. This is generally considered the most efficient HP-for-gold option in the game that can be used by just anyone (with UMD). That comes out to 1.364 gp per HP.
A pearl of power costs (spell level)2×1 000 gp. But a pearl of power is a 1/day item – a 50-charge item costs, by the guidelines,1 2½× more than a 1/day item. So by that metric, a spell slot costs (spell level)2×3 410 HP. Which is a ridiculous number, but it’s the closest thing to guidance that the existing rules provide. If you go below this number, the staff turns wands of lesser vigor into cheaper pearls of power.
The fact that it takes a lot of time definitely justifies a price reduction. Assuming you have all the wands you could want, and ten willing targets, you are adding 100 HP to your total each minute. 30 minutes for a first-level spell slot is bad; the 24+ hours for a 7th-level spell slot is completely pointless.
But when we are starting at the spell level squared multiplied by 3 000 HP, even if you drop it to 1% of that number you’re still talking about (spell level)2×(30 HP), which is dramatically higher than any of the other suggestions here. And it’s really hard to justify dropping the cost of pearls of power down to 1% of their cost.
Charged (50 charges): ½ unlimited use base price
Charges per day: Divide by (5 divided by charges per day)
For 1/day, that’s ⅕, versus the 50-charge’s ½. ½/⅕ is 2½.
More realistic: someone gets Fast Healing
As soon as someone gets Fast Healing (or Regeneration), this staff turns whacking them into free spell slots. Dragon shamans and dragonfire adepts can do it at level 1 (though they can only restore up to 50% of their HP; just have the cleric heal them back to 100% with all the free spell slots they can get). Various other options for it are available. Goodbye, one of the few somewhat-relevant limitations on spellcasters.
Conclusion: This is not a good idea.