One of the strongest symbols in Cathedral by Raymond Carver is that of the ten peso coin that Robert had. It is representative of the relationship that he had with Beulah. It is only worth something to him (it's cut in half).
The two men drawing the cathedral has often been a source of contention for scholars. One can assume that the author was perhaps offering the reader encouragement to 'draw' or 'take away' something from the work...even if it is just a sketch.