I'm not sure it's been studied, but I'd wager that every tax dollar spent on subsidizing public transit (or possibly even making it free) would deliver several dollars of gain. Physical mobility is one of the best predictors of economic mobility[0], and I imagine the poor would be able to find better jobs with fewer economic hurdles put in front of them.
[0] http://www.nascsp.org/data/files/csbg_publications/issue_bri...