Quote from Cache Landing:
For most equity option the "theoretical price is pretty much always the same as the mid. This makes sense because it doesn'treally matter which way you solve the equation, you will come up with the same number.
The SPX is a different story. The b/a spread is so wide and fluctuates so quickly that you really need to have an idea about IV first and figure out the theoretical value of the otions from that. Many times the mid really doesn't tell you much about what you might recieve/pay at any given strike.
Hey Cache...TOS is coming out with something you will love in their new release (a couple of weeks I think). You will be able to see what the price of the underlying needs to trade at to fill your option bid! way clever
very useful for those of us that throw in our bid on the SPX...we will actually see what the spx needs to trade at to get filled. We got to preview the new changes in Chi...very nice
It can get pretty time consuming figuring it out for all my positions.