I can't speak on Mother's Milk WTA. No offense to them or what they do (or to those that like it), but I will pass on them.
Both Aroma and Wholecig sell WTA straight-up. Meaning what they extracted is the complete alkaloid profile of tobacco and then it gets diluted down, just like pure nicotine would be handled before going into eliquid. WTA is, as has been noted, about 95% nicotine already, so when ordering a WTA at 24 mg/mL, know that 95% of that 24 mg/mL would be nic and only about 5% of that remainder is the minor alkaloids (they are "minor" after all). Now, 5% doesn't seem like a lot, but it can make a world of a difference to how the nicotine feels.
Aroma sells WTA without added nic, and they also sell WTA with added nic and flavoring. The result when adding nicotine to WTA, as you suggest, is just less percentage of minor alkaloids and a higher percentage of the major alkaloid, nicotine.
Wholecig sells WTA without added nic (even in their flavored WTA).
Is WTA expensive? Yes! If I had my druthers, I would have Aroma's on tap, but it's too costly for me to use considering I don't *need* it. If I needed it, I'd just cut out the more superfluous vape purchases (like silly, unnecessary driptips and such -- those prices add up!) I haven't had Wholecig's in a long time, but I think that the best possible way to judge which is best is to get both Aroma's VS WTA (WTA straight-up, no *added* nicotine) and WholeCig's WTA. For them to be compared, one would need to buy them at the same percentage WTA, and since Aroma only sells it at 24 mg/mL, that would need to be approximately the level you buy it at from WC -- that is unless you plan on diluting it. In which case, if you plan on vaping 12 mg/mL WTA, you could buy Aroma's 15 mL WTA at 24 mg/mL and add an additional 15 mL of VG (or whatever solvent you like) and end up at 12 mg/mL and you could just purchase WholeCig's at 12 mg/mL.
I've spent a considerable amount of time talking to the "creator" of WTA over the past year and a half and after having a lot of his WTA, and comparing the retail products to what he makes, I say that Aroma has the best product but WholeCig has the best price(s) and the best options (strictly speaking about WTA).
Note: I am actually kinda impressed with how well here WTA is understood. There was a time before when it was hard as hell to get a handle on this and we were all trying to figure it out for ourselves. Luckily the original chemist/vaper that brought this to life this still lurks about and is more than generous with helping to understand.
Congrats! 
