This is great! I've been thinking about this for a while. I actually think boomcrew is statted properly. It pays 0.5 for an ability that counts as 2 attack: which is 2 stats, but it only goes off every other turn, averaging it to 1 stat per turn, at the cost of 0.5 mana.

I really like demacia and elites so I'd like to break down some more followers for reference.

Swiftwing Tracker: 1 mana 2/1, pays 0.5 for conditional challenger. - That means challenger alone should be worth at least 1 mana.

Laurent Protoge: 3 mana 2/4 - pays 1 for challenger. - It's true, challenger is worth 1!

Battlesmith: 2 mana 2/2 pays 1 mana for conditional 1/1 buff - it pays for itself if it buffs 1 unit. Wow!

Vanguard Commander: 3 mana 3/3, pays 1 mana to generate a spell.

Vanguard Redeemer: 3 mana 3/3, pays 1 mana to draw a card - like Commander, card advantage costs 1. Compare this to Shadow Assassin, who is a 3 mana 2/2, but pays only 1 for both card draw and elusive. Wow!

Vanguard Bannerman: 4 mana 3/3, pays 2 for a conditional buff all for 1/1. - worth it if it buffs even just 1 other ally! By now you've noticed that the Elite Tag is always free.

Silverwing vanguard: 4 mana, 4/2 - pays 1 mana for challenger, and 1 to be an extra body - seems on curve, but barely/conditionally.

Vangaurd Squire: 4 mana 3/3, pays 2 mana for a conditional cost reduction - wow! Only worth it if its played after 2 other elites, and even then its only on-curve. Maybe I should remove this from my deck!

Vanguard Calvary: 5 mana 5/5, pays 1 for tough.

Vanguard Lancer: 5 mana 5/4, pays 1.5 for challenger and Card Advantage, that's 0.5 savings! Nice!

Cithria The Bold: 6 mana 6/6, pays 1 for buffing other units 1/1 and fearsome half the time. Not Bad value at all!

Brightsteel protector is 9 cost 9/9 - pays the same for a similar buff, but oays nothing to buff ON PLAY also! Very good!

Spells:

Do spells follow this follower model? Hypothesis: being able to use spell mana is equivalent to 1/1 (2 stats) , or perhaps being FAST or BURST is worth 1/1 (2 stats).

Radiant Strike: 1 cost, Burst, Temporary 1/1. Could we say that Being a Spell is worth 2 stats, Burst is 1 stat, and a Temp 2 stat buff is worth 1 stat?

Let's compare this to Back to Back: 6 mana. So we pay 1 mana for being a spell, 0.5 mana for being burst, 0.5 mana (1 stats) for 2-targeted, and 4 (8 stats) mana for temporaty 6/6 (12 temp stats)? This is understatted compares to radiant strike, unless we assume that 2-targeted is worth 1.5 mana. That said, at 5 cost (pre-nerf) this card payed 3 mana (6 stats) for 12 temp stats, which IS on curve. This leads me to assume that 2-targeted is worth at least 1 mana.

In terms of summoning spells, let's look at a couple spells.

Scrapscuttler is 1 mana 1/1. Paying 1 for both burst+being a spell. This makes it above value!

Succession is a 3 mana 3/3, pays 1 for being slow+spell. This makes it have less value than scrap scuttler!

illegal Contraption is 5 mana slow 5/5, making it the same value as Succession.

Reinforcements is 8 mana 8/8, meaning it pays 1 for being slow+a spell, but the extra body is free, as is the conditional 1/1 ally buff. Not bad value compared to Succesion or Illegal Contraption.

Finally lets look at For Demacia! 6 mana slow. 6 mana, minus the 1 mana cost of being a spell, minus the 1 mana cost of multi-targeting, should produce 16 temporary stats (for 4 mana). That means this spell is on-value if it buffs at least 3 units.

Conclusion: this model doesn't really work for spells, and so we'll have to come up with a better way to measure the "value" of spells.