Happy Inn soi 4. It's opposite soi 6 (the soi going towards BTS and Lolitas). Friend of mine stayed there a month ago and it was 600 per night. Clean rooms, A/C, hot water, TV of course. Visited the room and can recommend it.
Nanatai Mansion in the end of soi 4 I can also recommend, 1000 THB but w/ swimming pool. Huge rooms, new beds, flatscreen tv:s. 24/7 room service. They have renovated a lot lately. You may have to pay extra though if you bring more than one guest.