Since you seem to have some technical background could you share with us the necessary steering rudder force needed to keep a FAB-250 stable as it is accelerated by the rocket booster?
You’re asking about parasitic drag. It is negligible at low velocities and at low velocities you can’t steer for that very reason.
These missile concept approaches will not have propulsion present for very long. Typical A/A missiles are only boosting a few seconds. The rest is coast. They are not flying, they are coasting. The fin size is small for this reason. No lift is sought.
The guidance algorithm will face the problem of maneuvers based on targeting. The US used a concept of “compassionate targeting” where impact on a target is from a direction the algorithm was informed pre launch would minimize the splash of debris on a sensitive (hospital) facility. Point being, there’s not much steering required for high mass vehicles. They are largely ballistic and need only wind compensate.
So your question about fin force applied will vary with algorithm. If no request is made of the vehicle to do an approach from a side that won’t generate dangerous splash, that is more fin force than a direct, near ballistic approach.
From a general, Circular Error Probability perspective, you aren’t really doing much. You are seeking to reduce a purely ballistic munition’s error of 150 feet down to 20 feet. It’s not much steering. It was already pretty accurate to begin with, if programmed correctly and there is no hurricane blowing.