At my old place, about 100m2 I was on top floor and basically had no ceiling (it was just that 1-2 cm fake ceiling board, some black sheet and roof tiles). Aircons were underpowered, and when on, compressors were running most of the time, and when turned off, the place heated up to external temperature within 15-20 minutes.
Before all these power price hikes, my power bill was based on 5 THB/unit and was regularly 4000 baht up, reaching 7000-7500 in April/May hot period.
If your place is badly insulated, if outside compressors are old, and with 8 baht/unit price, the it would be possible to get to that figure.
Actually, if my electricity price was double (I'm in condo so paying EGAT rate, about half of yours), with aircon on most of the time, I'd get to about that amount as well, so I think the larger part of the issue is the 8 THB/unit cost. Then again, electricity price is rising every month and even the EGAT rate might reach that soon...