Sounds weird...$900/month (That's 31.600 baht) ... Can't be that much even if you are over 100 years old... (Except if you have already a lot of existing health problems...)
That's usually the yearly cost. Get help from a visa company and they can tell you the exact cost because this price is definitely not right.
If you are from one of the visa exempt countries (but depend on the country because few countries just get 15 days stamp on arrival) not necessarily. You get 30 days on arrival and for 1900 baht extending for another 30 days... Or come straight on a TR visa which gives you 60 days. The TR visa 1400 baht. So if the extra 500 baht matter TR visa is your choice if not just extend in the country. It's a maximum of a few hours in the immigration office.
that's changing for months... It was a case in the past. You just pay, get the student visa and that's it... Nowadays immigration coming in to the school, checking students, etc. And this is fact I'm studying thai myself and immigration just visited my class on Friday. I didn't say no chance to find a shady school where you can do this but much more harder than before.