As we had to redo all wiring anyway (renovation of a 80 y old house) and worked in stages it’s a bit difficult to do an estimation. Generally we found KNX is about +15%/+20% to comparable conventional wiring depending on the complexity (conventional wiring is cheaper for simple “one switch one light” situations but gets immensely expensive for more complexity - we found KNX was cheaper for some situations like “four different switches in four different locations all switching different combinations of lights”). In total around 40k € for a large house- that includes rerunning all wires, a few specialities due to age of the house and installation by a master sparky but no programming by them(did that myself - it’s not that difficult actually but takes a bit of time to get into).
The KNX wiring in theory could be done by a amateur as it is 24v only, but 240V needs a professional here. If we had done all KNX wiring ourself and let the sparky only do the 240V part (which in retrospect we should have done) we would have actually gotten out cheaper than conventional wiring, but I had no time to do so.
Of course the level of integration we opted for is far beyond what you normally put into a house - it’s a hobby more or less and we will not break even in terms of energy savings ever - but as we had to do something anyway why not do it right. Additionally it is heavily geared towards us getting older (e.g. we have motion detection at the ground level beside the bed - this recognises if you get up at night and now switches the bedroom lights on at 5% red so the wife does not wake up and then switches the lights on towards the loo. The whole routine is capable of recognising that someone has fallen or is unable to get off the loo)
It all depends on the brands you choose - as KNX has a huge spectrum of suppliers there is everything from cheap switches that are hardly more expensive than regular ones and top notch switches that cost 500€ each…we went with rather cheap but flexible ones.
A friend did some calculations with normal “off the shelf” smart home stuff like Hue, etc. and was 20% above what we payed for comparable level of integration.