Looks like you’re American, so it will be 800k. When you arrive, open a Thai bank account and deposit it. Needs to always be in for the 2 months preceding your 1 year extension.
If when you say immigration letter, you mean residence certificate, then the Bangkok Bank around the corner from immigration, was easy and almost no waiting to open a savings account. Did it 2 weeks ago. And be careful, ask the account mgr if deposits are not marked as overseas deposits. Can't remember which, but either SCB or Krungsri said they are not
I am on 90 day non o visa and opened an account with Bangkok Bank in Jomtien, Second Rd and Soi 7. Needed a resident certificate, which I was able to get with a Tm30 receipt(no contract).