Mine was $85 from Home Depot
http://www.homedepot.com/p/Siemens-20-Amp-6-5-in-Whole-House-Surge-Protected-Circuit-Breaker-QSA2020SPDP/202562776
I considered this: http://www.homedepot.com/p/Square-D-by-Schneider-Electric-Home-Electronics-Protective-Device-HEPD-HEPD80/203540660
but all positions in panel are used and circuit breakers are designed for one wire only - would have to pigtail it.
http://www.homedepot.com/p/Siemens-20-Amp-6-5-in-Whole-House-Surge-Protected-Circuit-Breaker-QSA2020SPDP/202562776
I considered this: http://www.homedepot.com/p/Square-D-by-Schneider-Electric-Home-Electronics-Protective-Device-HEPD-HEPD80/203540660
but all positions in panel are used and circuit breakers are designed for one wire only - would have to pigtail it.