you mentioned you thought it could be CCV related? have you ever had the CCV serviced? if not, i would get that done post haste. all four hoses and the valve itself. new intake boot and vac lines if necessary while you're in there. I have youtube videos bookmarked if you want to give this a crack. It's a 'level 2 or level 3' repair though.

while it is not unusual for an indy or dealer to sell parts at 2-3x the street price, $493 for a pair of oxygen sensors is, by itself, indicative of someone who either A) has NO idea what they are doing, B) did some serious half-ass research, which may be indicative of half-ass work, or C) is an unapologetic con-artist/crook. my vote is on C.

RockAuto sells the upstreams for $44 each (BOSCH 13477) and downstreams (BOSCH Part 15109)for $53 each and you could probably change all four of these yourself in less than an hour. Buy an sensor socket on Amazon for $8. O2 sensors are the first 'level 1' repair every BMW owner can make after air filter, spark plugs, oil change and coolant drain/fill are mastered. And i'd personally rather change O2 sensors rather than drain a DROP of antifreeze (I hate that stuff). So a DIY in this case is worth $800-$1000 depending on how much of that labor is for the sensors in the quote.

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2005 X5 3.0i - 71k mi (9.2018) -> 81k (9.19) -> 100k 9.21 -> 123k (8.25)
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SOLD : ( 2003 X5 3.0 - 177k mi (9.2018) -> 186k (9.19) -> 205k (9.21)
SOLD : ( 1997 328is Coupe - Hellrot Red
SOLD : ( 1988 528e w/ Bullseye s256 / MS2 Extra / GC Coilovers / Yukon Coils ~ 300+ HP