Okay, so like I advised, I made no early game purchases from Shopkeep. It doesn't really matter, you can't make enough money in the early game to really impact your purchasing potential given the high costs of later options, but might as well save it. That does mean that you can spend money on the things that give you relationship points (helping out Amber, paying for breakfast with Emily) without it impacting things, though.
Once the stat booster shop opens up, your budget matters. Unless you took Tiffany's money option, the most money you'll get will be from H.E.R.O. payouts. You'll get $3000 total for successfully hunting the Gorgon and Asura, and the S.I.N. ceremony infiltration pays out depending on the route you take. Assuming you did everything perfectly, you'll get $38000 if you stayed in the tournament and $49000 if you dropped out. I dropped out, so I had $52000 to play with. Actually, since I had left over from early game, it was $53060, but that doesn't super matter. Like I said, the extra isn't enough to afford anything I wouldn't have.
I bought the Spine and Vertebra from Shopkeep. I think that's a better use of the money than buying Power, because if you skipped Tiffany's money option for the Power option, you'd get +30 power and all Shopkeep's stuff combined only gives +15. The Spine is +3 Skill for $12000, and the Vertebra is +1 for $6000. Budget is down to $35000. That's exactly how much the Vortex Stone ($15000, I said $12000 earlier but that was me misremembering) and Raffle Ticket ($20000) cost combined. Probably that's deliberate. Having the Vortex Stone and Spine together unlocks an option with the Vestige that costs 1 Skill, but gets your choice of lore or a futher +3 Skill, so I think it's worth it.
And then at Shopkeep's Power shop, I spent the full 30 Power to buy the Shapeshifter's Heart and get +3 Skill. Maybe that's a mistake, we'll see, that's what save scumming is for.