That doesn't make any sense as far as the price goes.
In my current playthrough there are two level 7 heroes I can recruit. One is in an army with 2 mirror elementals (pretty weak units) and cost almost 500 gildars to recruit.
The other one comes with a very powerful lvl 4 Slag (the slag is a lot more powerful than the lvl 7 heroes it is with) and I need almost 1700 gildars to recruit that champion.
If I don't get the Slag why would I pay 1.7K ?