ATC 185S and 200S are the same. Other ones will have differences - 200E, 200ES, 200M are all a bit different. You are correct, it is highway robbery for that part, and you might find other parts are really expensive to source as well. What comes to mind is the metal seat pan for that bike - they are super hard to find in good shape, and when you find them, be prepared to pay! Restoring these old bikes is super fun and rewarding, but the pitfall will be the hard parts that are OEM only and people have found that they can set a high number on them and if someone needs it bad enough, they'll pay.
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