[swarmee]

aka, had the wrong premise. Taking low grade Mantles and breaking even on them isn't really what card grading/flipping is for. Sure, there's some premium tacked on to having it already sealed in a PSA holder: it can immediately be added to a set registry.

But you're doing at least three things wrong:
1) Focusing on issues that everyone collects, so everyone's been scouring for those players. Most people can spot the difference between a VG and an EX+ and know that there is not much upside in those cards you picked.

2) Buying at the low-end tier: card grading works the best when you can find $5 raw cards and turn them into $50 or $100 or $1000 PSA 9s and 10s. Why do you think that the collector here flipped out when his Nolan Ryan 1976 or so popped GEM MINT? Because his raw card became a card he could sell for $2000. Look through SMR and select some condition sensitive series and then try to find the cards that would grade high in those to interest the moneybag collectors. The ones with the exponential ramps like PSA 5: $30, PSA 6: $40, PSA 7: $80, PSA 8: $200, PSA 9: $1000, PSA 10: doesn't exist yet.
That's why a 1986 Topps Ryne Sandberg sold at auction recently for $2300. Seriously, 1986 Topps...

3) You're buying from a dealer at a convention. Browse through the online selection of 1969 Topps Reggie Jacksons on comc.com or ebay. Odds are, you'll pay much less and get a nicer card than you would have gotten at the show. I scour the eBay bulk lots for cards; lucked into a nice 1969 lot containing Mantle, Aaron, Rose, Killebrew, Carlton, Gibson, Carew, Kaline, all for a paltry $35. All I saw in the pictures was a sliver of the Mantle card, which I recognized. If all of those cards come back as 6s or better, I'm looking at a major profit.

Those are my suggestions. Your mileage may vary. There's a good book that gets hyped on Sports Collectors Daily that helps spells out how semi-vintage investing by grading should work, as well as which cards to send to PSA and which to BGS/BVG, based on centering and edges/corners. Mainly, it suggests focusing on Key Rookie cards of Hall of Famers like 1982 Topps Traded Ripken, 69 Reggie Jackson, etc.