You can actually buy the SPX by buying the synthetic in the option pit. it's also not a bad quote to look at while trading. The disseminated b/a has an issue with being stale. Some of those names may not have traded for some minutes. The synthetic in the crowd is a superior reference, because if it got too far out of line the upstairs basket traders would pick them off
Am noticing the BID and ASK from TOS differs from the BID and ASK from CBOE CSMI feed! Suspect either different sources OR perhaps could be due to delay difference (or both)! Observed difference during active trading seems to be within penny's of each.