Or cannibalize the parts for my own bicycle
They have to fit. Many ebikes have some proprietary part or another, and good luck engineering it together. You might get lucky, but you'll need to buy the junker to find out, and you're out the money without any recourse.
Batteries in particular are generally the single most expensive part and some of the proprietary ebikes will not accept a battery unless it sends the right signal - this is a deliberate action by the manufacturer to keep selling batteries and avoid the expense of supporting some random replacement they didn't design. It's both greedy and sensible for the manufacturer.
If you buy a junker with generic parts that you can swap out (some manufacturers do this too) you're left with the problem of 'why did it break?'
Are you in a position to make novel parts from scratch? From what materials? Many are, and I think this is great, but it's a question for you.
I can say there is one principle that applies when attempting to substitute my time and attention for money and certainty when pursuing a goal - I'd better have a lot of time and attention, and no one is going to do it for me.
Yes, I can imagine it all 'just working out' and 'first time!' - but that rarely happens if I stray from standards. In return for lengthy hassles pursing problems if I choose to persist I can emerge eventually with hard-won knowledge and experience. The cost is the 'lengthy' part and the 'uncertain' part.
There are very good reasons we gather in groups and work together and learn from each others - it costs quite a bit less to learn from someone's success than it does to work it all out from scratch ourselves. Every time.
As I move into retirement, I may take on learning more deeply by hacking into random ebike systems (among other topics), but in setting up my own, I have deliberately chosen to go with the best quality parts I can find that aren't strictly proprietary. In addition to being DIY focused, Grin has also shown that they are engineers who aren't trying to franchise and get rich - they are enthusiasts who want money to build more electric vehicles for humans. I feel very fortunate they are in the market, and I am happily paying them to keep on doing this as that's the best guarantee I can find.
My point is not that Grin is wonderful, my point is that they represent the best example I know of "you will get what you pay for". They happen to be a terrific resource to have. If you think from that perspective, you can evaluate what you really want and decide if they "cost too much". In my case, no. In your case, maybe yes.
You may not have as long a time horizon as I do, and you may also want to dive into learning the deep secrets of ebikes. I'm designing for 20 years (I know I can't rely on this, but I am - it helps me make choices).