Not particularly rare, but not particularly common. Basically, the bigger and/or more powerful the original engine, the more valuable the car is. So an 'A code' will usually be worth more than a 'C code', which is more valuable than a 'T code'.
Of course part of the value, though, is based on original options, and a large part of the value is based on the car's condition.
The 289 4v A code is a nice package, especially if its a GT. My dad had one (I've got it now, awaiting its restoration), and I remember him telling me how years ago he "raced" a regular c-code non-GT, which a friend had also purchased, around some highway onramps getting onto the freeway. In short, the race was decided rather quickly -- the firmer suspension and slightly more robust engine made for a totally different car, especially on the twisty bits. So the A code engine, especially with manual tranny, is a definite plus.
Now an S-code - a different car AGAIN - that is one which would make a muscle car into a MUSCLE car - just ask Steve McQueen :: So, like said above, the higher the horsepower, the more rare and thus higher the price.
As far as the GT models, any V8 - C, A, or S code- was eligible for the GT option in '67.
If memory serves (check previous threads on this), about 30 000 Mustangs were A-codes in 67.
Hope that helps .... my 2¢