Cruise control will not engage

The brake switch is a dual purpose switch correct?. Basically a two position switch. When bake pedal pressure is applied it disengages the cruise circuit and connects the brake lite system, please correct me if I am wrong on this assumption.

My mech. checked this and this is not the problem. He searched his data base and Toyota (Rav4) mechanic, they said to have Toyota do diagnostic. What will this cost?