Haha, yea interiors are a killer! But then, I try to do what I can to reduce render time - there are lots of things you can do to try and minimise this, like not using full RGB levels (white for example, I use 230 RGB and not 255). I try to keep reflective surfaces to a minimum and restrict the number of individual emitters (using multi-light and having a lot of emitter settings increases render time) but sometimes this isn't possible.
On average, for an interior that has many different textures/reflective surfaces and emitters (and individual components), it's approx £20 to render 1 image to 20 SL.
Anita