I have only rented condo's here ( 1 room, 1 bathroom + american kitchen style along with living room all furnished and with different amenities)
The low end condos can go from the ranges of 5000 bath ( these are normally the thai style ones) a month to 15000 thb/m depending on the area you living. ( If you living somewhere close to Sukhumvit/lumpini/ekkamai it will be quite expensive
Good areas to live in are Hui kwan, Suthisarn, Phra Ram 9, Chong Nonsri,Phra Khanong, On Nut. All near the BTS & MRT. And no so damn expensive.
As for bills: Water around 500 thb/m, Electricity - Depends if you are going to use the Air con all night long (or have an electric stove)- Around 1000 thb/m, Internet around 700 thb/month.
For food: Well thats your choice, you can choose the range of mobile food shops all over Bangkok that each "dish" costs around 20 thb, if you are a restaurant type, around 300 to 700 thb ( medium range restaurant ) or if Your condo has a fire plate you always can cook something at home. Now, there is two different kinds of supermarket here: The "national" range, such as Tesco's, with the vast majority of products from Thailand and Asia related, or the supermarkets aimed for ex-pats with a wide variety of imported European and American products. These last ones, are also the most expensive ones.
For transportation: Taxis, are well cheap ( the ones that got "meter" on top ) around a 100 thb per destination. Motorbike taxis might vary, but less than a taxy normally. And tuk Tuk's unless you are able to defend yourself with some thai, youll be ending up spending around 500 thb for a ride ( like most of the tourists do )
As for Bts & Mrt - Skytrain and Underground - Depends where are you going, but around 35thb per ride. Then you can have some short of an "oyster" card that you can top up. ( never had one, so i cannot tell you the thb's)
As for mobile phone: Going to MBK you will be able to get a nice deal for around 500 thb new thai number and can top-up in any supermarket or minimarket such as 7/11 with a 100 thb or 300 thb.
The apartments with two bedrooms are quite expensive for renting. What Thai people normally do, is that they use the "living room" as a second bedroom, with a sofa-bed or a inflatable mattress.
Total per month:
- This is what i spend normally a month -
*Condo fully furnished+pool+gym+guards+garden=9500thb month
*Transport= ( because i dont live near a mrt or bts) 3400 thb month - need to take a taxi then a train then a bike and back-
*Bills= 2050
*Food= - Got kitchen at home - 5000
*Going out = 5000 thb month.
Total:24950 thb/month ( this varies depending if im a bad boy or not on weekends )
Hope this helps you get an idea of what is going to be per month.
Ah forgot to mention that all this includes two persons.