NHF – new heroes for Heroes V 3.1.

10*(Hero Level) for a level 7 creature, and for each subsequent lower level, it's halved (rounded up).

For level 1 creatures. For level 2, it's halved, and so on.

Maybe someone can suggest something easier and simpler that would also balance with the stealing mechanic?

How much simpler can it get? I calculated it for the first level, checked if it works/doesn't work. If it doesn't work, I halved the number and checked for the second level. And so on. After all, the special ability works exactly like that, sequentially, checking from level 1 to level 7.

Added 3 minutes later
Well, other simple options are only linear. It's impossible to approximate the above formula (which is practically perfectly balanced with the average strength of the creatures) using powers of two.