Having a large choice of summoners is key. Most people don't have the money to have all summoners at level max but you actually don't always need them to be at max level to compete.
Some summoners are performing very well even when they're not at max level...for example, if you play in gold league, Kelya Frendul at lvl 5 is enough to compete even against the best teams.
Splinterlands is not only a good cards battle game, it's also a good money management game 😄