What do you mean "finding"? The AI would just return links or raw text of the cases. Reading the findings would be the same as reading any precedence. But the AI could weight the results, and you'd only have to read the high scoring results. If the AI got it wrong, you'd just refine the search and the AI would be trained.
To the cost. If it removed the need for one legal assistant or associate then anything less than the cost of employing said person would be profit. So if it cost < 50k a year you'd be saving. (cost of employing is more than just salary)
You can't validate that it is making the right citations by only checking the cases it is citing, and the rankings it provides of those and other cases. You have to validate the non-existence of other, particularly contrary, cases it should be citing either additionally or instead, which it may or may not have ranked as relevant.
To the cost. If it removed the need for one legal assistant or associate then anything less than the cost of employing said person would be profit. So if it cost < 50k a year you'd be saving. (cost of employing is more than just salary)