How much spending money would I need for a month in thailand?

If you go now you will get a double room with breakfast and air-con in those places for around 1200bht/night with Koh Samui being the most expensive resort.One px will get through about another 300bht/day on food and drink and a night on the lash will cost from around 100bht for a tourist beer or tail(big measures).If you can eat local your bills will be way less...more like 300bht per week,so there are many savings to be made.It all depends on expectations,personally I cant see the point in going away then roughing it.That's ok if you're backpacking or on a gap-year.An ok exception to that is internal travel.Flights are easy and regular between samui,phuket and bangkok but if you're staying for a month time isn't an issue,so I would recommend bus and ferry travel,it costs a lot less and you'll get a much better feel for thailand,and a sea voyage is such a relief from the heat and humidity even if it's only for an hour or two.It seems to me you've got the balance about right in your plans but dont be persuaded to stay in bangkok for a week,a couple of nights is plenty before you come home,one day to visit attractions(more than enough) and a bit of shopping on your last......I would recommend you book your first night hotel on- line just to be sure of having somewhere..you don't want to be shark prey when you arrive at bangkok feeling grubby and ragged check out sawasdee.com or kohsamui.org