Since all 420a and 4g63 engines use an interference design, you MUST change out your timing belt at the 60k mile mark. If you do not, you risk SERIOUS engine damage. If a timing belt breaks, your valves in your cylinder head get out of synch with your pistons in the block, and when they do, they can actually hit one another causing irreparable damage to your valves and possibly even your pistons.
Therefore, a timing belt job at 60k miles is necessary. Typically, the job is labor intensive taking mechanics a good 3-4 hours to perform...perhaps more or less depending on their level of experience with this engine. Parts can run about $200-250 as well. That'll including the timing belt itself, both pulleys, a tensioner bracket, tensioner, and water pump (on a 420a). Make sure and have all of the above changed out. Merely leaving out one of the above can have detrimental results in the future.