Lance Berkman has played the last two seasons in a park that favors pitchers. Despite this, he hit .295/.408/.533 in those 684 plate appearances, and owns a career line of .291/.393/.485 in this iteration of Busch Stadium over 110 games and 427 times up.
Being a switch-hitter helped negate the issue to a degree, as Busch is nowhere near as tough on left-handed power as it is right-handed. Berkman himself was a significant reason, though, as his power works almost universally: the average distance on his 31 2011 homers was 406 feet, and all but two of his homers would have been gone in at least two-thirds of the league's parks.
Because of this, Rangers Ballpark in Arlington might not help Berkman to an extreme degree, as he's already capable of helping himself out plenty, regardless of where it is he's playing. It will still give him an assist, though, as deep fly ball outs in other parks could be homers, and in the hotter months of the year, when the temperature difference in Texas can really stand out, the ball should just fly out of the stadium. Berkman is great on his own, but he's not so great that he'll get nothing out of the switch to such an extreme opposite.
According to Stat Corner, "extreme" is the correct word to use when discussing the difference between Busch and Arlington:
These numbers represent the percentage above- or below-average a park is in a given component, where 100 is considered average/neutral. Busch is fairly neutral for left-handers, while Rangers Ballpark boosts production by about 20 percent more on homers, and closer to 30 percent for other types of extra-base hits. The difference is even more extreme from the right side, and while Berkman will bat from there considerably less, removing an obstacle from, say, 100 plate appearances in his home park or so, does help the big picture still.
Obviously, this isn't being written to tell you that Lance Berkman makes sense in an AL-only league. It's more to help you gauge just where you want to get him, and maybe get you thinking about how much you want to spend. The production, rate-wise, is going to be there. If it was there in Busch, it can be there in Texas, even in a tougher division and as an older player.
What you have to watch out for is the cumulative production, though. Berkman logged just 97 plate appearances in 32 games in 2012, missing most of the season due to surgery on his right knee. He'll be 37 years old in 2013, and hasn't crossed the 150-game threshold since 2008. Without even taking into account his lost 2012, Berkman averaged just 134 games per year from 2009 through 2011. There's a very real chance he misses time, possibly significant time, as there is with any player of his age. Being the designated hitter should help to a degree, but just how much is up in the air at this point.
The upside is huge, though, so don't ignore him because of the inherent risk. Just be sure to go at him at the right time, and for a reasonable price, and make sure you have some kind of backup plan in order.