How to fix EVAP leak?

I'm planning to buy a 1997 Mitsubishi Eclipse RS. The "Check Engine" light is on and the seller told me that its cause by the evap leak. They run a smoke test a found out that the leak is somewhere in the canister. The mechanic the he knows told him that just replace the purge solenoid. How much will it cost? Does the car still good with this type of problem. Is it worth buying? Btw the engine was rebuilt. Any suggestion?