I get it, it’s not for everyone. The idea is to push you toward the newest knight instead of leveling the first one forever, but I hear you. The latest version now shows the price factor on each knight’s row, so at least it’s explained.
That doesn't work. Because of the extra cost mechanic, the ideal strategy is to only level up the first unit until level 200 (when there are no more multiplier bonuses) before buying the next unit. You already have an incentive to push towards the newest knight, which is not having any more multiplier bonuses after level 200, so the cost increase is redundant.