How much would it cost to replace the rear coil springs on my 97 ES 300?

The car's not sagging, but it bottoms out when there are 3 adults in the rear - which is rare, but not rare enough that I shouldn't get the work done. How much should it cost to have the rear coil springs replaced? And, do I HAVE TO replace the struts at the same time, or can I get away with not changing that too?How much would it cost to replace the rear coil springs on my 97 ES 300?
I'm not too sure about the rest of the question, but if your doing the springs, you might as well do the struts. Generic ones, if they'll fit your lexus, are cheap, and changing them takes little extra time if the springs are already off.