In Bangkok as I write this and can recommend two places, near to Khoa San Road:
1. My House Hostel, double rooms are 250 baht a night
2. slightly nicer is Lamphu House, doubles are 300 baht a night
They are both around the corner from KS Road on Rambuturi (or something like that). Sorry I only know double room prices but they should be a helpful guide.
Get a taxi from the airport from the pre-paid booth right outside the exit. Its very easy and should cost around 300 baht.
Hope that helps.