This doesn't account for $1200, but part of the difference is the shifters. The STI shifters cost rather more than the bar-end ones.

Who knows... the reason for the $800 gap in frameset prices is likely something that only Cervelo could really answer. My guess is carbon layup differences and/or associated labor costs.

For the complete bike, the S2's Ultegra brake calipers and crankset vs. the P2's Cervelo-branded calipers and FSA crankset are certainly a factor, as are the above-mentioned STI shifters.