Is that $1,500 USD for just a plug change or were they going to replace coils, hoses, serpentine belt, whatnot "while we're at it"?
I thought the $1,069 USD + tax quote I got for just the plugs from my Toyota dealer was a bridge too far so I tried out a locally respected mom and pop who charged $563 + tax after a $50 coupon using the OE spec Denso plugs. I had spoken to the owner with some leading questions and he showed familiarity with the job which inspired some confidence. So far so good after a month. The added bonus is I now have a less expensive alternative for other work.
The old plugs looked good but I'm led to understand the eye test is not a good measure of iridium plug wear. Ordinarily, I'm an "if it ain't broke don't fix it" guy, which would have probably been OK for a some time in this case as your guy suggested. But I reckoned that since I'm going to keep it for a few more years but not up to 240,000 miles and a possible second plug change I might as well get it done now.
By the way, a 2014 with 120k miles in respectable shape should trade at closer to $10,000 USD assuming the Car Fax is clean.