#Bitcoin Idea: Proof-of-Reserves-as-a-Service (PoRaaS)

The customer, who needs to “prove” their reserves, pays a fee and specifies a message to sign. The PoRaaS provider receives the fee and signs the message with a UTXO of sufficient size.

What would be a good fee rate for this? I’m thinking 1%. So if you need to prove you have 1 BTC, you’d pay a PoRaaS provider 1M sats for a signature.