I just got done making my own from 3/32" x 4' wide sheet rubber, by the foot, from a local industrial hardware store.
A bit of a project to make the patterns, to be sure.
I don't remember the cost, but shelling out $70 for already done sounds like a good deal.
What good are they?
1) Keeping wheel splash out of the engine bay is an obvious function, but here in dry Arizona, I went for years without, no problem.
2) Cooling system : as
@rolliges says, along with the engine under shield, they are a minor part of the cooling system.
I think the idea is that the heated airflow through the radiator is guided and channeled back over and around the engine and transmission.
This will be moderately hot, but less than the 194F (give or take) engine coolant temperature.
It has the benefit of being consistent in a wide range of conditions, slow or fast, winter or summer, giving better regulation the the cooling system.
"At working temperature" is what you want, not cooler is better.
Speculation - there may be some benefit at highway speed due to low pressure zone under the vehicle, pulling air through. Hard to say.
Has been observed - hot air re-circulation without these things at crawling speed. Not good.
The wheel wells are probably just turbulent and inconsistent, sometimes helping, sometimes hindering radiator airflow.
What a pain are they?
You need them out of the way for quite a few engine compartment maintenance chores.
Those damn little plastic fasteners are a total pain.
You can count on them breaking, lucky if they don't.
The fender holes are 6mm, which is mostly a Toyota only size, and always expensive.
I've spent a silly amount of time at Space Age Paints in Mesa, who stock every darn fastener in the Auveco catalog, 1000s I think, looking for ones that work and are re-usable.
Some 6mm parts don't quite fit.
Auveco #19490 works, and is supposedly re-usable (maybe, maybe not), but costs a ridiculous $1.71 each.
Pop, pop, pop, pop. There went $7 just to tighten the fan belt, or whatever.
All the different 6mm ones are expensive, unless maybe you go with the definitely one time "Christmas tree" type. Not sure there.
I got fed up and drilled out to 8mm, which gives a lot more choices, and much cheaper too.
Auveco #19239 works well for 8mm (once, for sure, maybe again, maybe not) and costs $10.46/25 (box)
You need a box of them on hand. If they don't break, they get lost.
In any case, getting them out with a screwdriver is an exercise in frustration.
You need a special tool to get 'em off quick. I custom modded some $8 thing from Harbor Freight. Looks great. They still break.
