I have lived in Thailand for 10 years and have never been asked to show a departure flight at immigration. However I have been asked by the airline innumerable times and only last December I had to sign a waiver stating that I would pay for my own flight if I was refused entry into Thailand.
A lot depends on whether you pay the bill direct or via the landlord/building owner. I rented a studio apartment in Bangkok and the building charged for the power and it was frequently well over 1500THB per month. Now I rent a two bedroom condo and pay the bill direct and it is rarely ever over 1000THB. That's having ac on all night in the bedroom and probably 4-6 hours during the day in the living area.