Maybe first visit Bangkok for a month or two and record what you spent and how you spent it.
"nothing fancy" is relative. Are you riding the bus and eating Thai food on the street and in food centers like the locals? Do you buy a beer in 7/11?
Or do you ride taxi, drink your beer somewhere in a bar, and eat farang food?
I have a farang friend who once told me that he just can't understand how some people can live here with less than 100,000 THB per month. Like: How do they do that?
He has wine for every dinner, gets 20k each time he walks to the ATM, buys the finest fish and cheese in Emporium, etc. He doesn't think he has a fancy lifestyle, just living an ordinary life.