Even some of the basics can get pricy. Complaints should be addressed to the people who build the cars. 
The shop has a certain hourly flat rate charge. This is multiplied X the labor time given in the flat rate manual and that’s where the labor charge comes from.
Some parts of the country are much higher on their labor rates than others and it doesn’t take long to start adding up.
At that age and mileage the car is due for spark plugs, platinum or not. Platinums do not help the car to run any better. Their only advantage is a bit more longevity as compared to copper core plugs. Considering you’re having to pay a bit to have this done using platinums would be the preferred option.
If a basic maintenance item like spark plugs have been put off because of the cost then I wonder about the air/fuel filters, PCV, service, etc. along with that timing belt issue, the latter requiring tensioners and a water pump also.
If the timing belt has never been done then it’s a time bomb at 12 years of age. This job will cost far more than 260 dollars and way more than that if it breaks.