Now, we know that spending a lot of money for a champion is worthless. That's because the price of Champion's equipments are very expensive comparing to units (while they give the same value). so, rather than we reduce the price, why not just make the champion's equipment different than the units?

Look at the game, or how people play the game

A party of champions? An army of 7 champions? WOW!!! They are not champion, but elite force, as champion is a champion. The best follower. Just like Lancelot and the Knight of Round Table, not a bunch of Elite para commando just like Navy Seal or something. So, rather than decrease the cost of equipment, we change the equipment by creating new special equipments just for these guys.

The first step is to separate the tech. For example, if you want to empower your unit, you can research the equipment tech from Warfare tech, but if you want to buy special armos for the Champion, you should research the Adventuring Tech. That's way you can reduce the urge of a player to recruit a lot of champions, but still consider them worthwhile.