I'm still confused as to how it is going to cost you 25k? My GTX330ES cost about 4.5k from memory.
Mode S ES transponders have only been available for (I think) less than a year and the prices are starting to drop as there becomes competition. At the moment the choice is Garmin or Trig. Soon it will be King as well (and maybe Avidyne?). Which in itself is a good reason to delay.
I think at the moment you can get a mode S ES transponder for about $3.5k. Anyone who has a King KT76A (ie most people) and fits the Garmin unit requires a new tray and a panel relayout (the Garmin is just a bit taller). I would prefer to wait for the King unit which might be available later this year. That requires new pin outs, but not a new tray (I think).
The Mode S transponder requires a new gray code altitude encoder for nearly everyone with a legacy mode C transponder. That will be about $1k
The big hitter is that I cannot use my current C129a GPS. So I need to fit a new C146a (WAAS) GPS. At the moment the only options are a Garmin 430W (while they are still available) GTN 650 or GTN 750. In a year there will be King and Avidyne options also.
The very cheapest option is a secondhand Garmin 430W which will probably be about $7k. A new 430W about $10k and a GTN 650 about $12k (all from memory).
The WAAS GPS needs a new antennae & cabling. So the interior needs to come out for the install.
The WAAS GPS (in my case) will replace a King Nav/com - so I will need a new CDI. That's about $2k.
The new WAAS GPS will also require that radio stack to be relaid. So more labour.
My guess is that 3 separate engineering orders will be required by CASA. At typically $350 each that's another $1k.
Total materials so far : $14.5k using a secondhand 430W.
Plus installation labour??? My guess is 40 hours @ $100 / hour = $4,000
Total $18,500 with a secondhand Garmin 430W or about $23,500 with a new GTN 650.