What's the best card to buy if we want power?
Renting cards for power is fine, but since liquidity on Splinterlands is so high anyway, there's no harm in actually owning a few hundred dollars worth of assets. You can get the rating itself with starter cards, so skipping variety in favor of rewards may be the best idea. So, how much do you currently need to spend in order to become a Silver III player with owned assets alone?
The answer is ~$250. No card is more cost-efficient than whatever the cheapest rare is on the market. That's because you can easily calculate this by using the cheapest card of all as a way to measure everything else, and that's the lowest price common card available. Today, the answer is $0.10, Gargoya Lion's cheapest market listing. Yeah, that card kind of sucks.
Collection power scales through rarity and edition. The newer a card is, the lower its collection power value. Chaos legion and the most recent Reward commons boost your power by 5. Rares boost your power by 4x this much. Epics boost it by 5x rares' power. Finally, legends boost it by 5x the amount epics add. In other words, rares are worth 4x a common's power, epics are worth 20x and legends are worth 100x.
If Gargoya is 0.10, then we expect rares are 0.40, epics are 2.00 and legends are 10. But is this true? No. Rares are just a tad bit above 0.30, epics sit at 1.7 and legends sustain 9. Clearly, commons are the worst possible choice if you want to build up collection power. Epics should be 5x what a rare goes for, but it goes for much higher than that - 1.70 instead of 1.5 (5x 0.30). Legends also disappoint compared to rares, since if a rare is 0.30, a legend has to be lower than 7.50 (5x and then 5x again) to be more cost efficient.
So we can safely conclude getting the cheapest rare is the best way to own assets that'll take you to higher leagues. 750 bcx of a 0.30 rare equals 15k power at a cost of $225.