Are you looking for new or used?
As
@BMBabe says there are plenty of used on Ebay. My understanding from realoem is that it's the same rear caliper on all Z3 models. However the rears are not shared with the E36 3 Series contrary to some listings.
Used are available for around £40 including postage which I think is fair. I could liberate the pair on my breaker, depending on how quickly you want them but I would want the same money.
A quick scan of new (which I suspect is what you are after) does suggest the rears are considerably more expensive than the front (and less choice of seller/supplier). I suspect this is because the front's are shared with the 3 Series and the rears aren't.
You may also want to consider refurbishment. I had a quote of £122 from
http://www.brakecaliperspecialists.uk/ recently for my fronts. Maybe the rears are the same sort of price. I don't think this sort of money is too shabby, especially to keep an OEM part on the car v a new non-OEM part.
Are you sure they need replacement? I'm only asking as 'could be doing with being changed over' doesn't sound completely certain!