Math below on the healing for the keys - note, its a VERY large boost to healing for any group that has 10 keys.
Assumption 1) all players have a key.
Assumption 2) Druid and Cleric have Lense of Divine Sight, and will always split healing across 2 characters
Assumption 3) Bard is 5th level
Assumption 4) Cleric uses Restore Spell on Bard for another Soothe Wounds
Example healing mechanic:
Cure minor wounds heals 3 total (1 base, +2 from LoDS).
It is split across 2 characters, healing each for +2, thus the total healing is 7 hp (+4 bonus healing due to keys).
Base, low equipment party Healing math.
A cleric has 10 healing spells. This is +40 healing compared to a party with no keys.
A Druid has 7 healing spells. This is +28 healing compared to a party with no keys.
5th Level bard has 2 healing spells + the one from restore spell. This affects the whole party, and is +60 healing compared to a party with no keys.
The base, low equipment party has healed +108 HP compared to a party without keys.
High Equipment Party, with healing focus
Assumption 1) Bard equips ring of spell storing, Carter's Tome, Crown of Expertise
Assumption 2) Cleric and Druid equip ring of spell storing and Crown of Expertise
Assumption 3) Druid equips charm of spell swapping, gaining 9 more healing spells
Assumption 4) Tome slot will prevent Gregor's Tome and Carter's tome from being uses together on Cleric and Druid
Cleric can cast 1 extra spell, and restore spell to Bard. +4 healing (bard covered next).
Bard can cast Soothe wounds 4 more times for +80 healing compared to a party without keys.
Druid can cast 11 more healing spells. +44 healing.
Total additional from high equipment party: 128 HP
High equipment Party (step 1 + step 2):236 HP
Variable healing per dungeon with 10 keys: 108 to 236HP.
Assuming $100 per key, that is 1.08 HP per dollar to 2.36HP per dollar.
Compare to: a cleric with no baton of healing and buying them a +5 baton of healing.
11 healing spells gain +5 healing, for 55 total healing.
Assuming 1250 for +5 baton of healing, that is 0.044 HP Per dollar.
Compare to: a cleric with no tome of focus buying a tome of focus.
11 healing spells gain +2 healing, for 22 total healing.
Assuming 100 for a Gregor's Tome of Focus, that is 0.22 HP per dollar spent.
Comare to: A cleric with no baton of focus buying the rare Baton of Focus
11 healing spells gain + 1 healing, for 11 total healing.
Assuming $5 for a Baton of Focus, that is 2.2 HP per dollar spent.
Conclusion: Buying 10 keys is a potentially better purchase decision for healers at a dollar per hit point than buying rare +heal items is. Optimal path is probably to buy the rare items first, though.
*edit, like, 5*
I forgot to put a charm of spell swapping on the cleric in the high level gear party. That is 5 additional healing spells (at the cost of bless and prayer, though). That is +20 healing, for a top end of 256 extra healing. I think a little bit to see what else I can squeak out of the key... maybe we can hit +300 hp healed?