Adam,
Since the SPX has many strikes (a feature that I really like along with the liquidity of this market), it is pointless to try to give you what the true prices should be for various spreads since the possibilities are nearly endless.
However, I will give you a few tips for determining the true value of the spreads that I hope will help. Xflat is obviously highly experienced here and will probably be able to add a few other pointers, as well.
1) NEVER accept the natural, unless a gun is pointed directly at your head from close range! The natural is out there to capture suckers who have a lousy broker or don't know any better.
2) Find a spread that meets your criteria. Then don't buy it for a day or two. Just watch the prices. Every hour check the midpoint for the spread and write it down. Write down the index values as well because noticeable movement will definitely affect the prices, but remember that most spreads will have limited movement because both options will change in value with index movement, although not by the same amount. You'll soon see that even the midpoint will often vary substantially. Of course, your job will be easier if the price of the index stays relatively flat, but you'll soon see that the price moves along with the index, but does vary widely.
3) It is often easier to trade a vertical instead of a complex spread. Complex spreads often need to be parceled off into pieces. Market makers like verticals because of the limited risk involved. They can also capture a little on both sides. Try your order in various ways to see if you can find a more attractively priced component.
4) Realize that the market makers have an advantage. They see every order from every broker. You will not often be able to trade exactly at the midpoint, but you should offer at that point to start your order. A matching retail order may have come in. Since the major function of the market maker is to be an intermediary, the market makers love to do little swaps between retail customers taking a nickel from one and a dime from another. This transaction involves no risk for them and is an easy $150 in cash profit (minus minuscule commissions) on a 10 lot. As Xflat has stated, this market is pretty big so there is some competition between market makers to collect this easy money.
5) If your order is a decent size (say 50 SPX), it really pays to have your broker get a quote directly from the floor. This quote will usually be much tighter than the bid-ask you see on your computer screen.
6) If you are working smaller items, you can get a pretty close idea of the true market if you check SPY options instead. They are very close cousins of each other. As an example, if you want a price on a 1100-1110 call spread, you would check the 110-111 call spread on SPY and then multiply the price by 10. Frequently, the bid-ask spread on the SPY is less than two cents if the options are close to the money.
7) If you have portfolio margin, you can use SPY options to hedge SPX easily if you dislike the prices you are being quoted (just remember to avoid expiry, and deep in the money options that have no time value-they might be exercised). This is not your first choice because of commissions, but if the price is right, it may work better.
8) Major increments (ex. 1100, 1125, 1150, etc.) are typically more actively traded. If these increments fit your criteria for purchase or sale, you may have better success with them.
9) Quotes for options that are a long way away from the money (such as March 950 puts right now with the SPX at 1110, or 1250 March calls) will typically have very wide bid-ask margins. The volume in these options is much lower, and matching retail orders is much more difficult for the market makers. That means that you will have a really hard time catching an attractive price for these options. Inventory issues will be a factor since one day more retail traders may wish to be selling than buying. The market makers are required under most conditions to make a price available even if they are not excited to do so. If you are selling to open, and under no pressure to trade, you should still offer to sell at the midpoint. Why voluntarily offer to be fleeced! Getting filled may be another matter.
Other tips, Xflat??
