Cost to install IFR GPS in a basic IFR 172?

Googling the group yielded little so here goes...

1979 172, overall average condition with mostly original radios and
wiring (one RT-385 recently replaced by a TKM radio, M1 Loran from a
few centuries ago)

The Cessna 300 ADF has gone flaky. The box tests fine on the bench
which suggests a wild goose chase may be in store to find the
gremlins. One of the two ILS approaches at my home field require ADF,
and a few fields I go to are ADF/GPS-only so I need one of the two.

My idea is to find a good used Apollo GX-50/GX-60 unit and replace the
ADF. No need to touch the Loran, it makes a good backup. I'll get
legal ADF and DME capability not to mention a moving map. Trying to
convince 4 other partners this is a good idea b/c it will make it
easier to sell shares in the future and even if they are VFR-only the
moving map is a big plus.

The local flight school says they've been spending an average of $6000
to do this with their birds at the local shop. Those of you who've
done this recently, any thoughts?

Bet,
-cwk.