Yes the trays for both diesel and petrol are now NLA. Remaining Audi stock in the supply chain is dwindling very fast, only 3 left last week, I think you are too late but ask. There are other aftermarket manufacturers but I hear they are thin and flimsy and not worth buying.
As to your other points.
The Audi price a few weeks ago was £59.97, it is only a bit of plastic, the £450 was a massive rip-off or a misunderstanding - maybe oil sump but I would be surprised to find still available, anyway always quote your model and year in a post that involves parts or model dependent other things, it just makes it easier for the person trying to help and avoids assumptions and guessing and easier. (I'm just as guilty as charged).
Yes you can safely do without the tray but it reputedly gives a little sound insulation from the engine noise and helps prevent leaks from staining your parking area to some extent, and I hear it is an integral part of front bumper support.
I cannot believe it is hanging on by one fixing, probably held by a fixing in each upstand into the chassis. I would remove, inspect and repair if possible and if repairable buy a aftermarket fixing kit set, £15ish for the million and one fixings. Having said that I believe a few are not available but Paul(Depronman)on here, offers a 3-d printed substitute.
If not repairable buy a low cost used one from one of the recyclers on here when available (well worth joining our Marketplace), but they will not post (item to big and liable to be damaged in shipping) and you must collect, either from North or South Wales or Is near Brighton?(Ami).
Welcome by the way.
Andy
Edit. Me and audifan typing at same time again!