Values are always park-dependent. Stanton is great in Rogers...in AT&T I think 10.5 is better spent elsewhere for a guy with about .250/.380/.540 slash against RHP on his card (before factoring in ballpark adjustments so in AT&T his numbers would be similar). He'll probably hit close to .200 against RHP in AT&T...no thanks.
Of course, values are always park-dependent, but I don't think I would scratch Stanton off automatically.
First, even though home team is AT&T, to take your example, there's still 81 games to be played on the road. When I do my ratings, I always consider that half the games will be played in neutral parks (BP=9/9 for both homeruns and singles), and half in a specific stadium (At&T for example). Also, I change a bit the pricing structure so that I have as many bargains and busts as in other contexts. The net effect is that very good offensive players in stadiums like AT&T tend to have a little boost in pricing--- expressed in other words, it's so difficult to have offense in stadiums AT&T that I'm willing to pay for players like Stanton who, despite losing 8BP, is still capable of generating 8.5 chances of direct homeruns (as opposed to, say, Ozuna, who loses his 6BP and is left with only 3.3 direct homeruns).
The net effect is that Stanton is still positive in the context I set up (home=AT&T, away=neutral).
Stanton is worth 10.57$ is this context---not a bargain, but not a bust either.
Abreu for the record goes down to 6.99M---more than 2M under his pricetag.
Kiermaier is a more difficult card to judge, since his performance is so abyssal vs lhp. If you create a combo with R.Davis, I have their combined performance at 7.24$ (a slightly better bargain since the cost is 7.02$).
So yes, Kiermaier/R Davis seems like a better purchase than Stanton, but not by much. And my ratings assume a 70/30 lefty/righty ratio---Stanton becomes the better choice at an increased level of lefty pitching.
And you still need to decide the exceeding 3M you are left with Kiermaier/Davis----if you're left to get a third reliever you don't know really need, than Stanton is the obvious choice.