It's an opaque field how's a client supposed to know this particular law firm is billing less hours than another would because they're using this software? Can't run the suit twice with two different firms blind to see which bills less hours.
You look at the firms track records, testimonials, results; etc., the same way you evaluate most purchases. If two firms have similar results and one is cheaper, then you choose the cheaper option.
What do you mean you know very little? You would interview the firm and ask them what the discovery process was like. You likely wouldn’t care about the gritty details of how they sorted the documents. More important would be the scale of the discovery, the outcome of the case, and the cost. Unless there was literally only one firm handling this specific type of issue, then it will always be possible to compete on cost.
