hi, i think you may be able to find a one bed with 700$s if you are not too picky about location and flats easily..but bear in mind that you need to pay minimum 6 month in advance or 12 months (1 year advance payment requested in general).
i don't know how much you were getting as INGO in other countries as PM, but i think you should ask similar amount + accommodation...If you are going to stay in Yangon, living expenses are not cheap and you will probably spend 1000 to 1500$ per month if you are going to eat outside.
Good luck,
cheers