My suggestion is really pretty simple. If your car is unmodified the factory spark plug will work well for you and cause you no problems. When you start to modify vehicles by using custom chips, tuners, modified timing curves, changing injectors, nitrous, blowers, headers, hotter ignition systems and the list goes on, there are advantages to using something like a Denso Iridium plug (brand not necessarily important, though I do like NipponDenso). You're correct when you say that they can run 3 times the price of the factory plug. A spark plug in reality has a very simple job to do. All major brands understand this task and do it equally well.
When you get into a situation where you are modifying the timing curve or pressurizing a cylinder then other considerations need to be looked into. The heat range of a plug as well as the gap will likely be changed. Some of the more exotic materials in spark plugs are of benefit in these circumstances too. When adding boost to a cylinder it is not uncommon to literally blow the plugs flame front out and cause a misfire. Hence hotter ignitions, narrow gaps and different heat ranges come into play.
If money isn't an object buy the Iridiums, they too will cause you no problems and you'll have the peace of mind that comes with knowing you have one of the best products available. If money is a consideration buy the stock plugs. After everything is said and done, if no other changes are being made to a vehicle you won't notice the difference in performance. One last thing comes to mind, should you have a vehicle that is an oil burner at times you can bump the heat range of a plug up a notch and help the plugs live somewhat longer before they become oil fouled.