Famas are worse than M4A1-S. Famas are slower to reload, give you a worse movement speed and do less damage. The have the same rounds per second.
AKs are slower to fire, reload and move with than the other two, but are more likely to do fatal damage from one shot.
Given these stats it seems you will have a marginal advantage chosing the M4A1-S. You'll be able to move and reload more quickly, and do more damage.
But I think the real problem is the human factor, at least how I play :D I'm always shooting at knees.
So to answer your question about value for money: the stats suggest that upgrading will be of negligable benefit. Stick with the Famas.