Actually... Somerville's rent has little to do with the Red Line.
Somerville property values SKYROCKETED a few years back. My in-laws bought their home in the late 70's for $23k, and now it's likely worth $800,000. And that translates to people realizing they can charge a ton for rent and get away with it. It's also near Harvard, MIT, Tufts, Lesley. And it will be even worse when the Green Line extends out to it over the next decade.
I actually used to live just around the corner from where you hang out. If you were to go up Summer Street (to the left of Domino's), there's a set of lights at Willow. I was on the next street heading towards Union Square, 4 houses down.
I'll definitely keep an eye out for any rental properties going up.
There's a place in Union Square next to my in-laws that doesn't have the best parking, but it's on the 85 and CT2 bus lines to Kendall Square.