[13lackGu4rd]

cheapest thug would probably be the black servant, a decent choice in the early game, and being ethereal is never a bad thing... but keep in mind that "cheapest" depends on quite a few things, such as:
1. your nation- this will determine how you can use your gems. for example, a nation with practically no death magic(such as Jomon) death mages might not be as valuable as heavy death magic nations such as C'tis(all eras) or Helheim.
2. indy mages- can fill in gaps in your national magic diversity, which in turn will connect with the 1st point about how you can use your gems.
3. luck- this comes in 2 forms. the first is random gem events(not too significant with Misfortune Scales but they do occasionally occur even under Misfortune2). the second is the magic sites you managed to find in your kingdom. the more income you have of a specific gem the cheaper it is for you to use it for side spells.
4. stage of the game- in the early game there isn't much you can do with death gems for example, besides a few items such as skull mentors and horror helmets. while in the late game it's all about tartarians...
5. your enemies- your enemies will determine what you need to react to, what counters you need to bring to the table. for example if facing Marverni than battlefield clearing spells such as Earthquake, Rain of Stones, Wrathful Skies, etc would be used quite often if you have access to them. if however you're facing MA/LA Ermor, C'tis, etc than Wither Bones and even Undead Mastery suddenly become your go to spells. notice that the gems you need to cast those spells(in battle, not from your lab) are different, and although each spell doesn't require many gems it piles up quite fast.