you need to be able to compare apples with apples. you can buy a suit from around $200 up... the $200 halensteins job will be very average, buy an $800 Cambridge and have it altered, it's good, or go and get a made-to-measure (or better still, Bespoke), and you've a different experience.
If the BMW batty is genuine aparrently they last a lo=oooooong time. You're paying retail, there's probably a full system test from the BMW dealer.
The mid-ground might be reasonable quality, should be tested in-situ, charging system test, provided with a warranty.
The cheap end will be possibly a lesser quality unit, perhaps bolted in, maybe they'll dispose of your old one. Maybe a lesser warranty, charging check if you're lucky.
like the suit, you pays your money and makes yer choice. Jeez it's Auckland, you guys are always driving, yeah? Maybe the distance to JC makes the $600 option worthwhile.
There's always the AA Battery Service, cheaper if you're a member. Not a forum sponsor though.
HTH