You can find commonly used alternatives on forums that specialize in your specific bike instead of buying the BMW branded items for routine maintenance.
Spark plugs, air/oil filters are items that I’ve sourced from OEM parts from Amazon (NGK plugs /Mahle filters). Save a little bit of cash, but really just for the convenience of having some spares on hand.
As for oil, there are also alternatives to the branded stuff. Just need to pay attention to what rating/certification that is recommended for your bike, and it will be fine. Even BMW changes manufacturers from time to time.
For peace of mind you can’t go wrong buying the officially recommended stuff, but as you’ve seen there is a bit of a premium when doing so.