You need to go back to Calverts site and read again. They're $439 a pair.
SPLIT MONO LEAFSPRINGS – Calvert Racing
One thing that hasn't been mentioned is that with Calverts split mono springs is that you can change the ride height simply by changing the rear half. If you order the wrong ride height they'll exchange them at no charge. I don't about the shipping though.
I personally don't like the look that SS springs give a car, reminds me of the 70's with N50's and air shocks. Yes, I ran them too.