From my research which is nearly for 4 years old now. There was not much price difference from doing the ATPL theory to just the commercial stuff. Also their wasnt as many outfits offering just CPL theory courses (distance learning). There are however the likes of Bristol (my choice any day) and Oxford who offer excellent and well established ATPL courses (probably because ATPL theory is in higher demand).
If study time an issue then I would assume CPL theory would require less due to less subjects, but if the notes are not as good etc then it may take a little longer. Catch 22
Good luck what ever.
Instructing is great fun
Mint