@Robin *******
if you use your own money, it should cost about 15.000.- THB for opening bank account, get the 90 days visa and the subsequent 1-year extension, plus a single re-entry permit
if the agent has to front the money, then of course it might come at a cost of 30.000.- to 36.000.- THB
You mentioned you got the 800.000.- THB. Then why would you need an agent for anything else but opening a bank account for you? In Pattaya you pay around 5000.- THB for this service.
The “change” to the 90-days Non-Imm-O visa you can do yourself, it costs 2000.- THB.
The application to the 1-year Extension of Stay costs another 1900.- THB fee. Plus you buy a single re-entry permit for 1000.- THB