My seatbelt caught in the door when I slammed it and it seems I'm having this issue as well. Pacific Motorsports in Portland quoted me $350 for the part (!) and "at least" two hours labor. Glad I checked here first. I'm gonna go get a dealer quote tomorrow just for fun, then go ahead and do it myself!

Found one at $109
BMW Door Handle Assembly Front Left Outer (X5) - Genuine BMW 51218243615OE | FCP Euro