The current subgraph for tracking Tornado Cash relayer fees (https://github.com/dmihal/tornado-cash-subgraph/) uses the MakerDAO V2 oracle to get the ETH/USD price.
Unfortunately, this oracle has been deprecated and is no longer being upgraded (price is frozen at around $2000). And unfortunately, the new MakerDAO oracle (0x64de91f5a373cd4c28de3600cb34c7c6ce410c85) only allows whitelisted addresses to read its prices (and it doesn’t appear that the Graph Protocol allows setting a from address for calls.
My suggestion would be to use the Uniswap V2 ETH/USDC pool as the oracle. While the V3 pool would be more accurate, the V2 pool is much simpler to implement as a price oracle (simply divide the two reserves).
However, I’d love to know if anyone has other suggestions for price oracles.