Mound kings(very fragile) and
banes don't cost that much, but they cost death gems which are very useful in late game to most nations in summoning tartarians. Sleeper is a tad worse since it costs nature gems which you'll need for both gear and Gift of Reasoning the said tartarians - certainly they can be used, but the opportunity cost is probably not worth it. Overall I'd say hire indep commanders, or preferably scouts as they are cheaper AND stealthy (your opponent might not see them coming as easily). There are a few mods out there - namely zlefins f/w thugs which add summonable commanders for fire and water paths for quite a good price. I haven't tested llamabeasts endgame mod which adds different kind of commanders too, but from what I gather they're meant to be more sc kind than thug kind so they cost a lot more.