Could be very expensive, if there are hundreds of feet of control wiring spread over the property.
If you want to get a vague idea of the expense involved for a do-it-yourself-er who knows absolutely nothing, add up the costs for a new controller, new solenoids for each valve (probably get them as complete valves) and new control wiring.
That is something like a "brute force" solution. A pro would electrically test the system, and replace only what needs replacing.
How can I look for a pro to test the system and tell em what needs to be done? What would they fall under in the yellow pages? I am guessing sprinkler system repairmen is a bit too specific?