To whom ? For some on the list it could be worth their life. To a hypothetical buyer depends on how effectively he can exploit/resell the data.

I suppose you mean how much it could be sold for ? The easiest/safest buyers is probably the company itself paid for by their insurance policies. Such policies will likely cover ransoms in the millions.

However with the current Twitter management it probably won't work, Musk may even not be paying the premiums given that he is not paying office rents or more likely be unwilling to negotiate .

For any other deal it is depends a lot on the seller and buyer. You wouldn't want to be drinking polonium after driving hard bargain with the Russians after all.

I talked to the hacker, he's asking for $50K

I may be wrong, but I believe this is information that was already available to you using the API. The only difference being that they have downloaded all the data and compiled it into an easy to use CSV. So I could see a lot of small-time spammers wanting a copy, but I don't see it being terribly valuable, like actual privative data might be.