Cost for Spark Plug Replacement

welcome! most definitely. anyone can replace the plugs, and 4 cost around 20 bucks. check with other dealers and local mechs if possible.
be careful of the 'induction', might be a wallet flush.

anddon't panic on the 120k service, it's not a hard and fast number. take your time and shop around, the plugs and coolant last a lot longer than that if you run late.

 

the plugs won't give out on you on the highway, and loss of mpg's can have so many causes, you have to about trying to determine the problem a bit scientifically, starting with the low hanging fruit and moving towards the more difficult services, i.e.tires model and pressures, driving route and temperatures, air, cabin, hybrid battery filters and fans, oil level, brake drag, throttle body and maf, and etc.

one area to consider is the egr circuit. it is a weak point in the gen3 engine, toyota won't acknowledge it, and most dealers don't understand it.
i hate to throw you a curveball on top of everything else, but this is important, as ignoring it can lead to a blown head gasket or engine,
there are many threads here, and a video @NutzAboutBolts . @Mendel Leisk also has advice in his signature.
unfortunately, you either have to attempt it yourself, leave it to a dealer who will charge a fortune and probably mess it up, or try to find a mech willing to study the issue and perform the service correctly.

 

Welcome Ashley; I've found that RepairPal.com to be an invaluable tool for people like you. So that you can get a reasonable estimate for the work done on your car at your local prices. You'll still need to watch-out for the shady shops; but at least you have a base price comparison for the work they are going to perform.

Hope this helps

 

Welcome, Ashley. Your location is a little vague, so it's hard to give advice about mechanics. But if you're close to Tampa, try calling Tampa Hybrids on Tuesday. (We're closed Monday for the 4th.) That way you don't need to be concerned about someone pushing bogus stuff like the induction flush. A half can of Techron in your gas tank will probably suffice.

I'm not sure of our price for plugs, but it must be way under $500 since we charge $600 for an EGR system cleaning and that's a ton more work. Coolant changes are pretty simple. Todd can tell you what's really needed and what it costs.

 

Here's @NutzAboutBolts video on 3rd gen (applies to plug-in Prius as well) spark plug replacement. There IS an absurd amount of disassembly involved to access the plugs, but even so, what you're being quoted is robbery.

The "induction..." bit should be waived; it's something you should research, not to be blindly agreeing to. Periodic removal of the intake manifold, cleaning of it, and the intake ports, and the throttle body is worthwhile. But leave for another day? DIY? See first link in my signature for more info. Also, here's another @NutzAboutBolts video, regarding the intake manifold cleaning:

^ I really doubt the dealership's "induction" service would doing this. More likely hooking up a bottle of solvent, running the engine near-stall for a while and blowing out impressive clouds of smoke from the exhaust. But not really cleaning.

And link to thread referencing all his Prius videos:

Nutz About Bolts Prius Maintenance Videos | PriusChat

BTW, with your miles, now would be a good time to look into Exhaust Gas Recirculation cleaning. Neglected, it leads to head gasket failure. First link in my signature again.

 

Actually, once you've done it a few times, removing the cowl only takes a few minutes. But it's still ridiculous that it should have to be done.