If you are coming and going at times that suit the airport express subway line (about 6:00 am - 10 pm), the Ibis at Sanyuanqiao is a hotel I usually go with. Much better than the airport Ibis.

At Sanyuanqiao station, the Ibis is about 50-100 m from the station entrance (behind the Novotel). Usually going for ¥300-350 a night. Nothing flashy, but has clean rooms and comfortable beds + showers. It's also got a couple of shopping malls with restaurants nearby and one can visit other parts of Beijing since subway line 10 is also at the Airport Express station. Airport express is ¥25 one way/¥50 return. Taxi from the airport to Sanyuanqiao is about ¥80-90 one way.