Undercarriage repair cost?

It depends what you mean by "undercarriage". I get the sense you're talking about the plastic under panels? If so: is it possible for you to safely raise the car, have a look underneath? By far the cheapest and most effective would be if you can get under there, see for yourself, get familiar with it, and purchase what's needed. There's various panels.

Just behind the front, painted fairing, there's a black plastic piece going the full width, about 10" wide. It's very prone to snag on concrete wheel stops, basically a design flaw. It can actually be accessed without raising the car, though a few inches of lift makes it a lot easier. It's item 52618 here: Front bumper & bumper stay for Toyota Prius ZVW30L - Genuine parts - Amayama

Then there's the main engine panel. It has a flap at the back that ostensibly can be flexed back for oil/filter changes, but in practice has a fairly brittle "hinge", that eventually breaks. Attached has info on it's fasteners.

At the front wheel wells there's liners, at the leading end of those are roughly foot square, horizontal parts. They've got bolts holding them to the wheel well liners, but they quickly evolve into rust lumps; you need to consider the whole thing as a single piece. It's item 53875A/53875A here (in illustrating 2 of 2): Hood & front fender for Toyota Prius ZVW30L - Auto parts - Amayama

Behind the engine bay there's another transverse, plastic panel, then an aluminum one.

Running down the size are roughly foot wide plastic panels,

In the rear, there's small plastic panels at the hinge points of the rear axle trailing arms. I found you need to go real easy removing the bolts on those; in the rust-belt it might be hopeless.

Then in vicinity of gas tank there's more panels, I kind of glaze over as to their details..Some are shown here: Rear bumper & bumper stay for Toyota Prius ZVW30L - Genuine parts - Amayama

I'm running out of steam trying to find the main engine bay under panel and associated; not sure Amayama still has them. Toyota USA has a parts site, but for the last fiew years, since it was revised, it's more-or-less unnavigable.

The attached pic may help with all of the above: Also, this link itemizes what there, part numbers for the panels and their fasteners: