So, the expansion is completely overpowered. :)
Besides, the effort required to upgrade monsters is many times greater than the effort required to upgrade a hero, so the cost-effectiveness is similar.
The conversation is about the comparability of the beneficial effects of different skills. And since it is impossible to compare them directly, an indirect scale is proposed - comparing the effect of an expert in a given skill with the effect of majors that are similar in functionality.
For example, a +6 sword is approximately equivalent to an expert offensive skill. Boots of speed are approximately equivalent to expert logistics. And so on.