There's quite cheap knx stuff on Ali Baba.
homeassistant
Home Assistant is open source home automation that puts local control and privacy first. Powered by a worldwide community of tinkerers and DIY enthusiasts. Perfect to run on a Raspberry Pi or a local server. Available for free at home-assistant.io
Loxone is not worth it lately. Their device quality has gone down, integration of other services is intentionally made difficult these days and overall support is lacking. I wouldn't buy them, even more considering that you are vendor bound and always need a central component/server for it to work.
KNX has a very broad pricing range - from very cheap to very expensive there is basically everything, there are countless PIRs for less than 100€ and some for over 400€.
A well planned KNX install isn't that much more expensive (10% on average)than a conventional install and the last Loxone install estimate I saw for a project was over 20% more than KNX.
Depends on what manufacturers and suppliers you want. Go with MDT (but avoid the GTs).
This is great feedback, thank you very much. So far I only talked to a loxone comercial, and another installer that works both with knx and loxone, and that person has recommended loxone. Do you know if loxone gives incentives to their installers ?
Yes.Some/a lot depending on how much you sell. And of course for an installer a walled garden is much much more convenient instead of a open standard. And it's much cheaper for them as they don't need an ETS licence, don't need an overview of hundreds of components and the certificates are cheaper/free as well - unlike KNX.