A win is 10 runs.
LaRoche has played roughly 8 full seasons, and has created 76 runs of offense with his bat. That's 9.5 runs a year, on average. This year, he's produced under 5 batting runs.
In 2011, Espinosa produced 2.5 batting runs. Last year, even with the post injury slide, he was worth -4.4 runs.
I think 1.5 wins is actually pretty generous if we're talking about a healthy Espinosa. LaRoche has never produced more than 21 batting runs in a season, so if you're saying there's like a 3 win difference between his bat and Espinosa's, you're basically assuming a career year from LaRoche and a one-shouldered Espinosa.