In the past year or so, while most archi tects have been struggling to develop plans for homes that could be erected for less than half again their estimated cost, Angus Wyman Macdonald has managed to pare building expense to the bone. The latest brainchild of this Virginia architect (whose no-frills approach to earth-sheltered construction was introduced to MOTHER's readers in issue 67, page 174) is the "Solar Arcade" . . . an 800square-foot, passive solar, earth-bermed residence that can be completed at a cost comparable to the purchase price of a trailer or a modular house.