Will the part 23 changes coming in August help to reduce the cost of IFR GPS avionics? I'm looking to upgrade the IFR avionics in a 1978 Cessna 182.
Unfortunately not: the purpose of the part 23 rewrite is to make it easier to certify a new aircraft type (not individual avionics or other systems) and it doesn't apply to existing aircraft. AOPA has a good overview, including this (emphasis in the original):
10. Are the performance-based standards applicable to all aircraft?
No. The new process of using performance-based standards is limited to new airplanes certified under Part 23, which would include airplanes with a maximum passenger-seating configuration of 19 or less, and a maximum certificated takeoff weight of 19,000 pounds or less.
To address the cost point directly, in theory if the part 23 rewrite creates additional demand for new aircraft, and if those aircraft use avionics that can also be retrofitted to existing aircraft, then the increased demand might reduce prices. But that's highly speculative.
I would say that the answer is a qualified 'yes.' The revised rules do not modify the need for type certificates nor does it affect the certificates of any existing certificated aircraft. So the major benefit will be when attempting to certificate a new aircraft design.
But the new rules can be applied to a modification to an existing type certificate. In fact the FAA normally insists that when you make a major modification to an existing design, the changes must be compliant with the current rules. This has historically been a burden because the rules kept getting tighter as time went along.
With the revised Part 23, the new rules can be used as the certification basis for an STC to an aircraft. The primary issue is that this isn't a cheap process regardless of the underlying certification basis. But EAA has been working to obtain STCs that can be used just for this purpose. In fact, they were able to obtain an STC for the installation of Dynon EFIS-D10A or EFIS-D100 as replacement for the primary attitude indicator. It should be pointed out that the Dynon equipment does not have a TSO or a PMA, but is verified against the recently developed ASTM 3153-15, Standard Specification for Verification of Avionics Systems.
EAA's STC has an approved model list. From EAA's site:
Currently, the Approved Model List (AML) includes the following aircraft series (including all variants on each type certificate):
Beechcraft Bonanza/Debonair, Beechcraft Musketeer/Sundowner/Sierra, Beechcraft Skipper
Cessna 150, Cessna 152, Cessna 170 Cessna 172, Cessna 175, Cessna 177, Cessna 177RG, Cessna 180/185, Cessna 182, Cessna 205/206/207, Cessna 210
Piper PA-24, Piper PA-28, Piper PA-32, Piper PA-38
EAA members can purchase the STC from EAA for $100, allowing the Dynon equipment to be installed in any of the AML aircraft types.
Hopefully the use of this type of a process will expand and provide for more options at lower price points than in the past. While it isn't a panacea, it will help.