Yeah, no. That won't work. The reason I based it on HP is because if I only need 5 HP per character, 20 camp supplies. Done. However, base it on a flat rate and it could cost you 40 camp supplies at level 5 for the same thing.
But, I can see that the cost I came up with could go in reverse. If 4 characters need 30 HP restored each, that's 120 supplies. Pretty hefty.
Wait. I know. Gosh! Why didn't anyone think of this before? They could limit Short Rests with Hit Dice. Ah! What a concept! Each character gets 1 Hit Dice per character level per long rest. Every time they Short Rest, they can spend as many as they want to heal. Once they've used up their Hit Dice, that's it. They don't heal anymore per Short Rest. You could still Short Rest to recover abilities, but not HP. No Camp Supplies Cost. Only Long Rest.
So, to summarize:
1. Allow unlimited Short Rests.
2. Short Rest allows Hit Dice use. Once Hit Dice are gone, no HP recovery per rest.
3. Arcane Recovery only able to be used during Short Rest, and reminder to use it would be nice.
4. Long Rest Cost 5 Camping Supplies per Character Level at camp (extras like Volo don't count).
5. Dialogues tied to Short AND Long Rests, and characters transported to camp when Short Rest also for dialogues and to show you ain't just healing somehow on the fly. You are pausing your adventure to rest.
Ah. So much simpler.