I think this is probably where this needs to go (prison time is interesting, but if we can't get it for financiers who profit off of crashing the economy, we'll never get it for ostensible negligence).
Establish standards for the value of stolen data. Something like the worst possible case. So for Equifax, which is potentially a gold mine for identity theft, fine them the average stolen identity cost (~$1300), for each of the 143 million records. 200 billion dollars (probably even 20 billion) seems it would be sufficient enough incentive to properly secure our data.
I also suspect we would need some kind of regular auditing, to ensure companies could afford a breach. Something like this would be a substantial drain on startups, as it would add another significant risk factor.
In the long run, what probably needs to happen is some kind of "data insurance", and we just expect that all companies working with our personal data carry it (or possibly even legislate that they carry it, similar to automobiles). It would make things easier for startups, who would pay much cheaper rates while their adoption was low, while also incentivizing them to limit their data collection to only what they needed.
Disclaimer: I work in the insurance industry, focused particularly on property insurance for disaster prone areas. Lately it feels like there are a lot of parallels between how personal data is stored these days, and Florida in the 90s.