These folks gave me a decent deal for a couple nights. 500 per night... I imagine they'd swing a decent deal for monthly. Free wifi / aircon / no hot water but the tap is warm enough.
Would be more comfortable if you can speak some Thai.
Hotel Name: Thanapa Mansion Address: 9 Phaholyothin 48 sub-soi 5 Phaholyothin Rd. Victory Monument Bangkhen
see room here, but contact direct rather than through agoda: Thanapa Mansion Bangkok Thailand - Best discount hotel rates