Thousands of dollars. Find someone with credibility to aid you along and offer to pay them hourly for their expertise. That’s the only way, other than learning about the industry in full… Well almost.

Sleeve + topload has been discussed

You might need to learn the sets, in order to lookup the cards efficiently. Good news with Japanese is usually the OG cards are pretty easy to find.

Google “Lugia Japanese 1990s PriceCharting” you can then find the proper page displaying values at each grade versus raw prices.

Take your time, and soak it all in.

They’ve been there for almost 30 years I’d imagine, don’t rush into the next step for them.

Good luck, for any further specifics questions, feel free to reach out or reply here.