Basically, sharing a room you're looking at £80 (split) if you want to spend two nights and £40 if you just want one night. Although there is also the option of crashing on people's hotel room floors if you want to drive the costs down further, but, that depends on if you want to pay for a hotel room and yet have to sleep on the floor.
If anyone else can find a cheaper hotel that's not a hostel, ridiculously seedy or in a crappy area, feel free to do your homework.