I did a quick search on eBay, and this is what I mean by a used key:
https://www.ebay.com/itm/12429386604...sAAOSwZtdfMeel
But I'm not sure if the 433 MHz will work for you. For some reason, I think it is 315 MHz. Something to look up somewhere, or maybe somebody else knows that.
I guess 7 years later, the prices have gone up and availability down, vs. what I paid back in 2014.
To go with the 3x eBay used diamond keys, I bought 3x valet keys from thebmwminipartsstore as you listed. Great deal, service, etc. When I got mine, the only options they had were diamond or valet.
Here is a thread about my solution, back when I needed to replace a couple of the batteries. Things still working great there.
https://xoutpost.com/bmw-sav-forums/...r-options.html