On the other hand, you may be able to do some of the work yourself, as
long as it is supervised and approved by an IA.

That is not true. An A&P can logbook enter a minor modification.



An IA would have to fill out a form 337
and submit it to OK City.

Only if your A&P is not skilled enough to avoid a paperwork blizzard.



We did everything
by the book and the FAA approved the changes. Once you see the paper
trail and detail work that is needed for such a job, you'll understand
the cost drivers that go into equipment installation.

Once you understand that being paid $$ an hour for a paperwork mill is a lot
easier than being paid the same $$ for sweating greasy wrenches, you will
understand why a lot of mechanics choose to do their "work" in an air
conditioned office with a pen. Hey, it's only your money, right?




Units on ebay run from $300-$600, and some come with a
yellow tag, which would be needed on the form 337 entry to prove
airworthiness.

The yellow tag means nothing other than that the unit met its internal
specifications at the time it was tested. It has absolutely nothing to do
with being airworthy.

Jim
A&P, IA