When we’re looking at vanilla minions, there’s a formula for value:
Total stats = mana cost * 2 + 1
Taunt would be worth 1 additional (which is why senjin shieldmaster is a 4 mana 3/5, not 4/5), divine shield counts as attack (so for 3 mana with divine shield you’d get a 3/1 minion, because 3+3+1 = 2*3+1 =7), etc.
So for example, a good for 4 drop could have 9 stats total, distributed as 4/5 or 5/4 (but generally, health is valued a bit more than attack, so you’d prefer 4/5)
However, since we don’t have vanilla minions anymore, it gets harder to evaluate the stats over mana ratio for value
What we have to do now is dissect a card to 2 similar ones and try to go from there
So for example, we have Alexstrasza 9 mana 8/8 minion which sets a hero’s health to 15. Since we got 16 total stats, which is worth more than 7 but less than 8 mana, we’re paying around 1,5 mana for the effect of setting any here’s health to 15, right?
Contrast that to INFERNAL!, new warlock card, 4 mana 6/6 taunt which does the same, but only to your own hero
Obviously, since you’re only doing it to yourself, so you would gain a half of those 1,5 mana for the effect, let’s say 0,75 mana, rounded up to 1, which means that for 4 mana we should get around 5 mana worth of stats
In conclusion, the card should have stats not higher than 11 total, taunt included, so the best it should be is 5/5 taunt or 4/6 taunt…
…definitely not 6/6 taunt.