Hmmm. I saw a thread at 3DBuzZ about moving components numerically, somewhat similar to the way one may use the channel box for objects. I Haven't actually checked it out myself, but it may work for this situation.

Well, just checked out the solution given and it doesn't seem to be of any help in this situation. Regardless, here's a link.

I do however have a solution that should work for you.:bandit:
After you've selected faces and hit the extrude button, a number of things happen. Amongst them, a new node gets created for the object. This has fields including LocalTranslateX, LocalTranslateY and ditto TranslateZ.
You can simply type numbers into these fields to get an exact extrude.

e.g -
Create poly sphere (pSphere1)
Select faces and hit extrude, but don't move the manipulators - Your channel box is now filled with all the fields of the newly created node (polyExtrudeFace1)
Now, drag the blue arrow and watch the field LocalTranslateZ.



The only thing I note, is that the orientation of the local-handles on the face seems to be related on the orientation of the face. The effect of this, is that you must allign the face's normal with one of the x,y,z axii.

For example,
1)create a polysphere with 8 divisions each way.
2)rotate it around the Y axis by 22.5deg
3) Try extruding a couple of faces one by one, paying attention to the orientation of the manipulator handles.

Notice how only the faces that are at 12, 3, 6 and 9 oclock move as expected when the handles are played with?

Lemme know if you need more details, or a better explanation.

Simon.