When I bought the cherokee it had 2 NAV/COMs (both old Narco). Radio
worked on both, no NAV. Both nav heads were dead, too. And an A/B
switch. In 1992 I had the entire center stack removed and installed

1) PSEngineering 6000 (shoulda gotten the one with the markers, but
didn't) ($700)
2) UPSAT SL30
3) Overhauled TKM MK-12 slide-in replacement for the Narco

The NAV/ILS head was replaced with a TKM LED indicator ($600 IIRC) and
is tied to the SL30. No more mechanical stuff. Downside is it's
not really that easy to see in bright daylight.
I didn't replace the 2nd nav head. I'm not IFR yet but learning.
I'd rather have that hole used for fuel flow and engine analyzer.
That's the next purchase. I think that will be more practical
at this point than the GPS. But I need a job first.

Unfortunately the airplane is at the shop for annual along with all
the books so I can't provide the actual cost. But it's now 3 years later
and altho labor costs have gone up, equipment prices are down.

The big cost was the wiring. I needed harness for the SL30 and all
the wiring re-done to get rid of the A/B switch (the *original*
equipment from 1968 when the cherokee was built!) for the PS6000. I have
working ADF/DME that was also tied into the PS6000. And a jack into
the PS6000 for music input. When I fly solo, I don't listen to tunes.
Safety issue, to me.

To get a panel GPS requires an annunciator. But since the PS6000 is
installed properly, it will be reasonably straight-forward if & when
I get the GPS. The hard work has been done.