350 000 VND
Depends on how many Ampere-hour the battery has. With a price of 350K I would
AT LEAST expect a 7Ah battery, but 9Ah should be possible within that price range as well.
Vatgia is as always a good source for prices:
I would not pay more than 10k, MAYBE 15k if it was in the middle of the mechanics lunch break for the work needed on a standard bike.