Dumb question - 

If a rollup switches out DA to some low cost DA solution, is there a major difference in cost savings if it’s ZK vs Optimistic? What’s the best way to think about the differences there?

assuming a "honest" centralized sequencer + decentralized fraud proof system (i.e. anyone can submit fraud proofs) over the long run, optimiums will be cheaper, simply because they do not submit zk-proofs for finality (which can have variable/fixed gas costs) but rely on the "challenge period" for finality

moving things @availproject / breaking things @deq / @chelseafc 💙

in general, a validium construct is more optimal because of the reduced "challenge period" considerations + there is no notion of an honest or dishonest sequencer *unless* the proof system is flawed, valid proof = honest (and rarely, proof system borked) what this also means is that the gas costs can be predicted

over a long period of time, it might make a lot of sense to pay the premium for validity proof verification for better UX/DX (in terms of cutting down typical challenge periods from 1 week -> 20-30 minutes on beefy hardware)

assuming a "dishonest" centralized sequencer + decentralized fraud proof system (i.e. anyone can submit fraud proofs) over the long run, optimiums will be more expensive, because current fraud proof systems are interactive and require multiple transactions over a period of time to reset the state of a rollup on L1