Tablets are coated so that they disolve when in your system. If you cut them then coating is not on 50% of the pill so bear that in mind. 8ths is not really practical.
Why not just but generic finasteride, its cheap and does not compromise the tablet by cutting it and you get recommended 1mg. Those on 1.25mg by using quarter increase sides potential without increasing effect by relative amount so 1mg should be considered max, studies show less than 1mg is still very beneficial. You could always cut the generic finasteride in half to save further money but I would take 1mg once a week or so to keep DHT inhibit levels as high as poss.