The hidden cost there is that the risk of complying with the legislation remained entirely with you. Even the best specialist research LLM still might easily have hallucinated or made some other sort of error which resulted in it giving you confusing or incorrect advice - and you would have been the one held liable for following it.

Whereas with real legal advice, your lawyer will carry Professional Indemnity Insurance which will cover any costs incurred if they make a mistake when advising you.

As you say, it's a reasonable trade-off for you to have made when the alternative was sifting through the legislation in your own spare time. But it's not actually worth very much, and you might just as well have used a general model to carry out the same task and the outcome would likely have been much the same.

So it's not particularly clear that the benefits of these niche-specific models or specialised fine-tunes are worth the additional costs.

(Caveat: things might change in the future, especially if advancements in the general models really are beginning to plateau.)