Let's say your old 1200BTU unit takes about 1000 watts per hour, and the new one 500, you are saving half a unit per hour per a/c.= Bht 2.5 (in simple numbers). On for 8 hours a day = Bht 25 = Bht 600 per 30-day month.
Cost of new Inverter a/c unit is around the Bht 20,000 mark. 20,000/600 = 333 months.
Change these figure to suit your situation.