My car is worth about $1000 but is in great shape.  I don't know if I
should replace the skid plate that recently broke off.  The auto shop
says that the plate itself costs $140, not including labor.  But the
guy who took the broken one off says that it's not really necessary. 
I asked if debris or water would get up there and damage anything, and
he said that there are vents in the skid plate anyway.  If stuff was
going to get up there, the skid plate wouldn't completely stop it
anyway.

So my question is: is it worth it to replace the skid plate for more
than $140?  I want the car to last because it's a great car, but I'm
unsure if the skid plate will make a difference.  If it won't make a
difference, if everything that the skid plate guards will be just
fine, then I want to cancel my appointment to replace the skid plate.

If you can justify your reasons that would be great.  If you have any
links to data to support your reasoning, that would be great too!  I'm
basically looking for an answer from an expert who, preferrably, has
data to back it up.

thanks!

Hi automator and thanks for the question.

As with most car maintenance/repair issues you have 5 choices.
1.Do nothing (often a false economy)
2.Repair it yourself but with non-genuine parts
3.Repair it yourself using genuine parts
4.Get it professionally repaired using non-genuine parts
5.Get it professionally repaired by a Volvo dealership

Depending on the nature of the problem and your budget you must choose
between the above.  I consider myself pretty good at car
repair/maintenance and can take an engine and gear box out
replace/repair whatever, and replace it usually within a day depending
on the problem.
My personal opinion is that ‘things are on cars for a reason’ and
often if you choose to ignore the problem things might be ok for a
while but then one night when it´s pouring with rain and you are on
that country road.......we´ve all been there!

The main purpose of the skid plate on your Volvo 240 is to protect by
deflecting road debris (stones, glass, nails etc) the cables that run
towards the front of the engine and to a certain extent the oil sump. 
It is not essential to the running of the car but it does protect your
car from potential damage.
If I were in your position I would choose number 2 in my list above. 
The part is not essential to the running of the car and so (in my
opinion) a non genuine part could be used.  Also the part should be
straightforward to replace requiring the minimum of tools (probably
only a socket set)  If you use the car in off road situations then of
course it becomes more essental and you should consider an aluminium
one.

Below are some links which should be of use to you, including a
non-genuine replacement for $25.00 instead of the $140 as was quoted
to you.

I should also add the disclaimer that there can be no substitute for
getting the professional advice from your local Volvo dealership.

Good luck with the replacement and if you found my answer helpful
please do not hesitate to rate it, and if you have any doubts
regarding my answer just ask for a clarification.

THX1138

Non Genuine part:
Skid Plate/Splash Pan/Protector Plate $25.00
http://www.fcpgroton.com/volvo240other.htm

Aluminium alternative:
“The original plastic sump guard or 'belly pan' fitted to our 240's
tends to be something that is prone to breaking and probably wasnt
designed to handle 20 or so years of use.”
http://www.geocities.com/ashvolvo/sguard.html

“The wires to the alternator and oil sender are normally routed under
the front of the engine where they are exposed to heat, oil from
leaking seals, and road debris if the belly pan is missing. Add to
this the flaky wire insulation found on some 240s, and often you get
instrument lights that don’t work or light up like a Christmas tree
under certain conditions”
http://www.geocities.com/motorcity/speedway/4006/240mods.html


Search strategy used:
Personal knowledge and:

"volvo 240"  Pan
://www.google.com/search?hl=en&lr=&ie=ISO-8859-1&as_qdr=all&q=+%22volvo+240%22++Pan&btnG=Google+Search

Excellent, detailed, cited references, and complete answer! 
Everything I asked for.  Big thanks!!!