I like picking up foils for Cube, specifically cards that I feel will be in my Cube for a while and/or are likely to jump in value in the near future. So far I've ebayed for a promo Lily (basically the only card I REALLY wanted a foil of), and promo versions of
Evolutionary Leap and
Harbinger of the Tides b/c I could get them each like 5 dollars cheaper than they're worth and I feel both of them have a lot of appeal in other formats (Leap for EDH, Tides for Modern Merfolk). I don't think there's much else I'm looking to pick up promos of in the near future, maybe
Abbot of Keral Keep if I can find it under 15. I've lost some money on some promos I've hunted for, but many of them have also turned out pretty well (KTK Dig Through Time, FRF Tasigur, etc.). It's pretty stable from what I've seen, most foils retain value pretty well and I can dump/trade them away later if I no longer need them for about the same amount. I normally do this only for rares/mythics though, since it's easier to find random foils of certain commons/uncommons in random binders or for super cheap off Amazon.
If you don't care for foils, I'd just wait a bit for the pricier cards to drop and try to actively trade into what you would want in the meantime. From what I've seen from a year of managing my Cube, a lot of cards tend to drop 2 weeks after release unless they see significant Standard play. If you see any cards that look like they'd have a home in Standard, then I'd try and pick them up early before the spike (if they're at a reasonable price). Or just pay attention to any big SCG Open or whatever that comes up, see which cards are seeing play, then quickly buy them before spiking. It usually takes a day or two after a good SCG showing for stuff to spike. I remember watching PT M15, saw
Perilous Vault had a good showing, bought like 8 of them at $3 and traded them in for $7 a piece two days later at LGS. Stuff spikes REALLY quick after a big event.
Look at TCGPlayer for certain pricier cards, usually you can find certain vendors selling for pretty cheap.