If you can solder you might try fixing the old fob. You already had it apart to change the battery (which IIRC needs to be soldered) so should be easy enough to take it apart again and check the solder at the switch contacts(and the battery). Your original battery might even still be good. If you still need a key a new one from BMW is the way to go.

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1988 325is (purchased new) sold
2004 X5 3.0 2005 X3 2.5
2008 X5 3.0 (new to me)