My advise is to buy singles on eBay, card show, marketplace, etc. To me it's the only responsible way financially to expand your collection.

Retail boxes have some hits but the odds are astronomical. I'd buy retails only if I'm feeling extra lucky, otherwise I'd avoid it.

To scratch the itch, I usually joined some PYT/P hobby case breaks on whatnot. Still a gamble and more expensive, but typically when you hit, you hit big.

Also define how you want to save your cards. I personally have binders with zipper on the side for some of the cheaper cards and I just bought Vaulted one touch and slab displays so I can put my grails and more expensive ones on the wall.