I had looked into this myself, but ended my quest when the reality of the cost hit me. The flagged coins are EXPENSIVE! You can pay as much as $600 for a single PR70 (cameo) coin. I saw some complete flagged sets of PR69 that were going for about $850 on the low end. I've attached a web site that show prices... I have zero affiliation (never bought anything) with the site and only found it during my search.
I cannot see the expense in slabbed, let alone the ultimate "70" slabbed coins in a specific holder. The proof silver quarters are great, highly collectable, usually of high quality and reasonably priced in the government yearly holders.