Portland has lots of pedestrian-friendly neighborhoods – check out these posts on my blog along with the discussions in the comments: “Best Thing About Your Neighborhood” and “Top 5 Portland Neighborhoods for Car Free Living.”
In general the further out you get from downtown, the more affordable housing becomes. Some neighborhoods I would recommend that have that combination of inexpensive housing and walkable amenities are Roseway, Hollywood, Montavilla, Kenton, Arbor Lodge, St. Johns, Sellwood, Multnomah Village. In addition some of the suburbs are less expensive and have very walkable downtowns, including Oregon City, Milwaukee, and Hillsboro.