I changed my plugs at around 50.000 miles and they were perfectly fine, then again at 80k miles and were perfectly fine again but i wanted to switch to Denso iridium. No difference at all in fuel consumption or engine power. Have them check yours before you buy them, yours should be fine too if the engine is running well.
Still, 300 pounds is a lot, what do they change for that money? Oil, all 3 filters and a aux belt is no more than 60-70 pounds. And 60 pounds for iridium spark plugs...so more than half is labor? For what, a one hour job?