What is the value of a silver streak 5mm pellet gun?

I believe you are asking about a Sheridan .20 cal (5mm) air rifle.

They started to make this rifle in 1949 and stopped in 1990 ( sheridan sliverstreak model ) During the 1990's the name changed to C9 sliverstreak and C9 bluestreak.

The value is determined by the current condition of the rifle. You did not give any such information , therefor I can not answer you.