I know how tempting is to go to peakmonster, find the Kron the Undying, rent it for a whole week, and just watch it win games for you. And don't get me wrong, this strategy works, but I'm engineering and I'm always looking for ways to get my job done in a more efficient way.
The free market
The problem with this overpowered strategy is that it gets popular, and because of the high demand of the market, the price skyrocket.
I have been searching for ways to climb the ranked while spending the least amount possible.
Find your edge
If our main concern is the high demand for the popular OP cards, we must find underpriced cards, by that I mean cards that are stronger than the price it shows on the market. The trick here is to use some Economy theory to get an advantage on the market.
Because some cards are easy to get, their price goes down by the market pressure, and because their base stats don't make up for a large selling price. But Splinterlands allow cards to be leveled up, so a horrible common card when leveled up can beat even the stronger monsters, by pure brute force.
Let me show you some examples.
The Minotaur Warrior is a pretty common card that has over 1500 cards listed on peakmonster, just by itself its base stats doesn't do much harm to their enemies.
But we can see a Lvl 10 listed for 9,50 Dec/day, 1/10 of the price of a lvl 1 Kron. And the base stats at lvl 10 looks like this:
Of course, Kron will win every single battle against the Minotaur, but my point here is that you can make a strong deck on a budget by getting powerful cards without spending tons of Dec.
Invest in your summoner
The max level of a monster is affected by many things, one of them is the summoner level. Don't forget to save some Dec for renting a level 3 or 4 summoners, otherwise, all your monsters will still be level 1 in the battles.
Wrap Up
If Dec isn't a problem for you, go for the strongest characters, they really worth the investment. But if you need to make up something out of nothing, then I would go for the cheaper higher level cards, besides worthing more than its current price, those common high-level cards are also a great way of increasing your deck power, allowing you to rise to higher tiers, always spending as little as possible.