At zhongguancun there's an 奥力 which should cost you 2000 per year. They start at 4000 usually, but just tell them no. There's some place north of shuangyushu in the basement of the community center that also has a big pool and should run you cheaper.
Haosha or Haotai or whatever it is over there is probably the cheapest at 1000 or less a year, but i hear it's pretty dirty. Aoli is the nicest one over there, but it's a small pool.
You can swim at renda fuzhong, big pool but it's stacked to the brim with filthy people because it's so cheap
The gym that occupies the top three floors next to huayu shopping center probably has the best pool, but it's pricey. I think they started at 4000 or so per year and I didn't bother negotiating a price.