I have a pair of rear struts for a base boxster for sale. They've got about 10k on them of light street use.
I upgraded to full coilovers a few weeks ago and no longer need them... if your boxster has 100k or more on it, you should replace your struts to see just how great the car was when it was new.
New? $209 each from Vertex Auto.
Mine? $150 plus hard cost of shipping for both.
PM me if you are interested.