There are several ACTV bus routes between Mestre and Piazzale Roma, if you find the taxi cost prohibitive. (Alternatively, but more complicated and slower, is taking the short, frequent train ride between the train station on Venice "proper" (Santa Lucia) to the Venezia-Mestre station and then taking a bus or taxi).
A large number of the people who work in Venice live in Mestre, and commute by bus and/or train. I stayed there on one trip, with a friend who worked at a hotel and whose family were among the many long-time Venetians priced out of the islands. It's not the hell-hole that some people would have you believe, just pedestrian and uninteresting, especially when compared with the wonders of Venice.