Let me explain.
Ok, it is well known that using a combat maneuver without the appropriate feat can result in disruption because of the AOO:
Unless otherwise noted, performing a combat maneuver provokes an attack of opportunity from the target of the maneuver. If you are hit by the target, you take the damage normally and apply that amount as a penalty to the attack roll to perform the maneuver.
Ok, well that obviously interrupts the maneuver. Would it be possible to ready an action to interrupt the maneuver like you ready an action to interrupt spellcasting?
Stated simply "I hold my action in case that shifty looking monk decided to try and grapple me, and stab him if he does". Would such a held action be capable of having the effect of subtracting from the CMB like the AOO does?
I think that by using RAW interpretation, it does not apply a penalty to the attempt like an AoO does, but RAI is fuzzier. The line "If you are hit by the target, you take the damage normally and apply that amount as a penalty to the attack roll to perform the maneuver" is a bit open to interpretation, though I think it does imply that it is referring to the AoO exclusively.
That being said, as a GM I'd allow it to do so. Seems pretty fair to hold your action to disrupt it, much like spellcasting.
An attack of opportunity “interrupts” the normal flow of actions in the round. If an attack of opportunity is provoked, immediately resolve the attack of opportunity, then continue with the next character's turn (or complete the current turn, if the attack of opportunity was provoked in the midst of a character's turn).
Ok, so an AoO interrupts the normal flow actions, meaning it happens before the attacker completes their action.
Unless otherwise noted, performing a combat maneuver provokes an attack of opportunity from the target of the maneuver. If you are hit by the target, you take the damage normally and apply that amount as a penalty to the attack roll to perform the maneuver.
So, AoO damage amount is added as a penalty to your CMB roll.
Ready
The ready action lets you prepare to take an action later, after your turn is over but before your next one has begun. Readying is a standard action. It does not provoke an attack of opportunity (though the action that you ready might do so).Readying an Action: You can ready a standard action, a move action, a swift action, or a free action. To do so, specify the action you will take and the conditions under which you will take it. Then, anytime before your next action, you may take the readied action in response to that condition. The action occurs just before the action that triggers it. If the triggered action is part of another character's activities, you interrupt the other character. Assuming he is still capable of doing so, he continues his actions once you complete your readied action. Your initiative result changes. For the rest of the encounter, your initiative result is the count on which you took the readied action, and you act immediately ahead of the character whose action triggered your readied action.
You can take a 5-foot step as part of your readied action, but only if you don't otherwise move any distance during the round.
Initiative Consequences of Readying: Your initiative result becomes the count on which you took the readied action. If you come to your next action and have not yet performed your readied action, you don't get to take the readied action (though you can ready the same action again). If you take your readied action in the next round, before your regular turn comes up, your initiative count rises to that new point in the order of battle, and you do not get your regular action that round.
Distracting Spellcasters: You can ready an attack against a spellcaster with the trigger “if she starts casting a spell.” If you damage the spellcaster, she may lose the spell she was trying to cast (as determined by her Spellcraft check result).
Readying to Counterspell: You may ready a counterspell against a spellcaster (often with the trigger “if she starts casting a spell”). In this case, when the spellcaster starts a spell, you get a chance to identify it with a Spellcraft check (DC 15 + spell level). If you do, and if you can cast that same spell (and are able to cast it and have it prepared, if you prepare spells), you can cast the spell as a counterspell and automatically ruin the other spellcaster's spell. Counterspelling works even if one spell is divine and the other arcane.
A spellcaster can use dispel magic to counterspell another spellcaster, but it doesn't always work.
Readying a Weapon against a Charge: You can ready weapons with the brace feature, setting them to receive charges. A readied weapon of this type deals double damage if you score a hit with it against a charging character.
So you can ready a a standard action, a move action, a swift action, or a free action. It also interrupts the normal flow and would occur before the grapple action is finished in your scenario.
As RAW, you do not add the damage from a readied action's attack as a penalty to the CMB roll. If it were a spellcaster attempting to cast, then it would add to the concentration check, but that is because of how spellcasting in melee works.
I would not let you add the readied damage as a penalty to the CMB roll. The readied action cannot be an AoO, andif it is a Monk trying to grapple then they likely already have Improved Grapple (of course they might not).
Now, you could ready a grapple or trip maneuver to interrupt and then the opponent would be forced to (if you grappled) A) Break or reverse the grapple or if you tripped them B)Get up from prone triggering an AoO.
So, long answer short, a readied action attack does not add a penalty to the CMB roll, only an AoO does if successful.