Bloomin eck Zane... That's a big bill mate for what it is.
I'm obviously in the wrong career
One simple question I would ask would be is:
"Ok... which coil pack is it then 1,2,3,4,5 or 6?"
If they have checked properly, they should actually know or are they are just guessing?
If there is a hesitation in the answer or they simply can't tell you, then you know it's subject to taking what they say with a pinch of salt...
A faulty coil pack will normally show as a fault thus throwing up a misfire code, its a monitored circuit, a spark plug won't as there is no sensor monitoring them.
There's your clue
It's not neccesarily true that all coil packs can go around the same time but in all honesty... "If it aint broken, don't try an fix it"
Change only one if it is indeed faulty, the others could last many more tens of thousands of miles with no hassle.
It is a fairly easy DIY job to change coil packs and spark plugs yourself btw with good tutorials saving a fortune.
6 plugs can be had for around £40
A good second hand coil pack from Ebay about £15-25
If you dont have any tools at all, then they can be bought to complete the job for no more than £40 including the clic R pliers for the intake pipes, spark plug wrench, a socket set and even hex allen keys...
I would consider certainly DIY mate... If you have any issues, I can help you along the way but that guide is pretty easy to follow.
Its an easy check to see if a coil pack has failed by removing the connector plugs to the front bank coils one at a time between start ups.. If the engine note changes and the misfire gets worse while each one is disconnected, then you know they are working fine... Change the plugs while you are on, there is no sense in removing things twice
Next, remove the plenum chamber as per the guide and replace the rear plugs and while you have the rear coil packs off, swap them over with the three good ones from the front bank that you know were working ok...
Now you fit the plenum chamber all back together and move back to the front bank and fit the original rear three coil packs in the front bank again and start the car up... If the misfire is still present turn off, now repeat the process as you did for the three front coils until you find a faulty coil pack... Something tells me you wont actually get a misfire though since you changed the spark plugs anyway