From: NE Pennsylvania, USA
My two cents on this, for what they are worth....
Fortified status gives infantry defense an x8 modifier, and I can see this for prepared fortifications, along the lines mantill mentions, but IMO this is too high for a unit in a non prepared fortification hex.
Also, entrenched in a prepared fortification hex, should be worth more than entrenched in a non prepared fortification hex. Another example, entrenched in mountain, urban, urban ruin, etc, should be worth more than entrenched in open.
So, what I would suggest (lol.. I tend to suggest <g>), is separate modifiers for entrenched/fortified status and terrain. In other words, two modifiers here instead of just one.
Hmmm.... there is also the case of a hex being less than fully 'entrenched' status. Could use this as a modifier to the entrenched bonus instead of a chance that the unit can become entrenched. I have noticed I can move a unit into a hex that is partially entrenched, and then if I dig in and do not become entrenched, can undo and try again until I get that entrenched status (yeah, tried to turn off that level of undo to be fair but, that was in another game).
The thing is, like mantill, I have seen infantry get to entrenched in one turn and, seems like more often than not get to fortified status on the next turn. Hmmmm, that may have been in mountain terrain but then again, as things stand now, entrenched in mountain terrain, doesn't buy you anything.