The theoretical value of an option is merely the output of a mathematical formula, of which there are several. In order to use one of these formulae you have to input several stochastic variables, and therefore there is no such thing as an absolute theoretical value. It is just an estimate depending on which pricing model you use and what you estimate the inputs are for said variables.
As to TOS, and I am a fan of them by the way, their theoretical value is useless. It is just the midpoint of the current bid/ask. I constantly see TOS traders put in spread trades for ridiculous prices based on midpoints rather than a correct theoretical value. Midpoints are affected by 1 lots on the bid or ask, but theoreticals are obviously not.
An example may help: the SPX July 1455-1465 Put spread is marked (based on midpoints) at 1.05 while the 1460-1470 Put spread (which has to worth more) is marked at .85. You need to know that they are both worth about .85 to .90, based on current vols, in order to trade them.