What is the value of a browning 20 gauge automatic 1970 model?

Value is too difficult to be accurate without seeing the gun. Condition which includes the amount of original finish and original configuration will determine value. Condition and age is also a factor. Auto 5's with recoil pads (other than Mag 12's) poly-chokes and compensators will reduce value. Recommend taking it to a gun shop for appraisal.