well, crystals themselves are more expensive than wood and stone

and wood and stone are found everywhere and are easier to guard, and by the time you have the choice to build champions or angels, you usually have enough stone and wood, but not enough crystals

By the time you can build them, you either have enough of everything or nothing, and there are almost the same amounts of resources on the map (usually), not just wood and ore. It's a different story if you have a secondary hero with the Miner skill, but that's an extreme case. In general, for comparing prices, it's better to take dragons and golems, which also require ore, but only ore, so it turns out that champions need three resources, not two. That's why they aren't cheap.