With this graph from Riot SapMagic and my experience from playing games, I believe tier 2 units need to be looked at.
They should be made available earlier, maybe right at the start of the game.
Currently, tier 1 units are not only half as expensive but also immediately available and the player will focus on leveling up those units from the start. By the time the player has a level 3 tier 1 unit or close to it, his little legend will be level 4 or 5, at which point tier 3 and 4 units are also available and are relatively easy to level up to level 2. There is little incentive to pick up a tier 2 unit others than for group synergy.
That means, with the combination of the steep gold cost, limited availability and competition from better units, tier 2 units stay at level 2 and defacto become the worst units in the game compared to level 3 tier 1 units and level 2 tier3+ units.
If tier 2 units were made available at the start of the game, I believe the higher gold cost (twice tier 1's cost) is enough to balance out their strengths relatively to tier 1 units.