Great work!
To make L2 scaling (i.e. stateless or state-minimized contracts with data stored off-chain and only a 32-byte stateroot stored in the contract) competitive with storing data on-chain using SSTORE, the cost originally suggested by @Recmo was “100 gas per word” or 3.125 gas per byte (see summary post here and proposal here).
The number is based on the size of a merkle proof assuming a depth 32 binary trie and 32-byte hashes (total bytes = trie_depth * hash_size = 1024 bytes). At 16 gas per byte the data cost of a proof would be 1024 * 16 = 16384 (brought down from 1024*68 = 69632), but 16384 is still not competitive with 5000 gas for an SSTORE.
Proofs can be batched/de-duplicated with a varying amount of reduction (depending on where the accounts are in the trie), so in some circumstances 16 gas per byte might be cheap enough to be competitive if proofs are batched.