Yes. 12,5000 baht plus 100 baht for a 90 day report should you choose not to do it online.
But they're busy so you'd need to go a few weeks before the extension expires.
Not sure what you mean about 1,600 baht bank charge though. Are you referring to opening a Bangkok Bank account, if so that's gone up to 4,700 baht. However if you're here already on extensions then a residence certificate should get you a Bangkok Bank account at a friendly BBL branch without having to pay anything.