Those are good, although iridium ones tend to last more.

I'd say $120 parts and labor for platinum plugs was OK. It probably would have cost the same for iridium plugs at a different mechanic. And I would not go to that same mechanic again since they put in the wrong plugs anyways. Using the wrong spark plug type can actually cause problems because they all use different voltages to provide the spark. Iridium spark plugs use less voltage compared to platinum and copper. The fact that the owner's manual states to use iridium plugs suggests that the ignition system is optimized to run with iridium plugs (less voltage), nothing else. So the use of platinum plugs may hinder your engine performance depending on whether the platinum plugs are getting the voltage they need to spark properly.

As I said earlier, I think they both work fine. It is just the matter of longevity and price and I the mechanic did not overcharge you. I always do my own research before going to a mechanic to work on my car to possibly avoid some issues. 

Personally-wise Ive had some sales people at my local auto parts store try to sell me up on the iridium plugs for honda/toyota engines. Of course the service manual will list iridium plugs as OEM but you have to think a little harder of how your car is running now and how it will be running in the next 30k miles. So many 20yr old cars will come down with some kind of parts failure that affects the A/F ratio. If you run too rich/lean for awhile after the problem comes up you will ruin the more expensive iridium plugs sooner. At that point it doesn't really matter that the engine would have *theoretically* ran 90k miles on expensive plugs because you toasted them at 20k and the extra money is burned away into the atmosphere.

If your car is running very well with lowish miles I would say go for the higher end iridium. If it is up there in miles/age I would say go for the lower end because they will likely last as long as they need to before some other problem comes up. My 2c