How dare they give me extremely good value for my money? $20 for an entire expansion when it'd usually probably cost me double or triple in terms of money or time? This is an outrage! I ought to sue!
... Okay, putting the sarcasm aside. Oh well, you can't craft the one card you want for your faction, that's unfortunate. Yet this model is better for many, many more people.
I'm terrible at math (software engineer who's bad at math... hah) but I think I can try:
1x Common = 40 Spirit, 1x Rare = 100 Spirit, 1x Epic = 350 Spirit, 1x Legendary = 900 Spirit
Spirit cost of Ancient Bonds cards per faction = (40*6) + (100*3) + (350*3) + (900*3) = 4290 spirit
Spirit cost of all faction cards = 25740 spirit
Neutral cards = (40*3) + (100*21) + (900*3) = 4920 spirit
Spirit cost of expansion = 30660 spirit
One core set orb is worth on average about 220 spirit, so to get enough spirit for the entire expansion you'd need to buy approx. 139 orbs. As you can probably tell, this would cost 13900 gold (as opposed to 3900) and way more cash than $20. I didn't double check any of my math (probably why I'm bad at it!) so I could be wrong. I think this is pretty accurate, though.