But first - do you have ADS-B OUT already? If so, here's cost-effective:
G5 as an HSI
GPS 175 using the G5 as indicator
All the parts needed for the GPS
This removes the existing DG, keeps your KX configuration and adds gps and HSI. Cost under $12,000 which includes installation, depending on the shop. Takes about 2-4 weeks depending how busy the shop is.
Yes, you must keep the vacuum system for the AI but you want cost-effective, right?
Next question - are you happy with the current transponder? If so, there are "add ons" to make it ADS-B OUT compliant. Much more cost-effective. Probably under $3000 and it includes installation.
Where are you located? A good, and I mean GOOD, avionics shop should be able to cut a new panel (not the plastic overlay) and re-route all the wiring, except that's going to cost. The 3 quality major shops in Colorado all offer custom panels. Another thought - have the panel you want cut and have the avionics shop re-arrange parts to match the panel. This will require the avionics shop and the custom shop to be in close communication to make sure everyone understands where all the parts need to go.
I've been doing upgrades for the past 3 years....the first was a new ADS-B OUT transponder - $5K in '18
In '19 the A&P installed the G5 AI (under $5K)
This year, avionics shop did the G5 HSI and GPA 175 - about $12K.
So yes, even low end will run about $25K overall. I just did it piecemeal so the pain to the checkbook wasn't as noticeable.