Cost to install IFR GPS in a basic IFR 172?

An IFR GPS *with a current database* is a leagal substitue for an ADF.

The requirement for a current database, if any, comes from the Approved Flight
Manual Supplement. Not all AFMSs have this requirement, but most do. You have to
read the AFMS for -that- airplane. Many UPS GX50/60 installs used boiler-plate
AFMS verbage that does not include the requirement for a current database.
Instead it requires only that the PIC determines that the procedure in the
database matches the procedure as currently published, or something like that.

Furthermore, an IFR GPS can be a legal substitute for an ADF for many
operations, but not all. AIM 1-1-19:

"The GPS Approach Overlay Program is an authorization for pilots to use GPS
avionics under IFR for flying designated nonprecision instrument approach
procedures, except LOC, LDA, and simplified directional facility (SDF)
procedures. These procedures are now identified by the name of the procedure and
"or GPS" (e.g., VOR/DME or GPS RWY 15). Other previous types of overlays have
either been converted to this format or replaced with stand-alone procedures.
Only approaches contained in the current onboard navigation database are
authorized. The navigation database may contain information about nonoverlay
approach procedures that is intended to be used to enhance position orientation,
generally by providing a map, while flying these approaches using conventional
NAVAIDs. This approach information should not be confused with a GPS overlay
approach (see the receiver operating manual, AFM, or AFM Supplement for details
on how to identify these approaches in the navigation database)."