Around here I wouldn't pay any more than $100 for a good usable as is SM420. Likely you can find good ones for well less than that, they are plentiful, strong, and less and less desirable now that we tend to drive fast and like/need OD transmissions.
Rear axle is worth about scrap value, and you should find some free ones if you ask around.
If you find one that someone has already and recently rebuilt/restored it should be worth slightly more than the value of the new parts.
Grigg
