i will assume you are talking a D2ZF AA spread bore 1972 4 speed. i seen them sell for as little as $40 unrestored, and $450 restored.
you have to remember the 71 boss carb is an autolite, the 72 and later are motorcraft, and althought they appear similar, they are some differance, that are unique to the boss 351 carb. and as far as rarity ford made 1806 boss 351, and they made atleast 6800 ford and mercury cars in 1972 with 4 speeds that would use the D2ZF AA carb. so ford made almost 5 times as many of these carbs, and atleast 58700 1971-1974 Q code ford and mercury cars that used a carb almost identical to your D2ZF AA carb, most were automatics, and some more 4 speed models for 1973, with different engineering numbers.
but to answer your question it depends on condition, if its complete, ( linkages , choke parts) and weither it still have the correct tag
