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).