Only the first 74xx bikes are considered 'rare' and you'd be hard pressed to find a junker going for less than a semesters tuition.
Other than these^^, and being very general, you have 3 other groups:
The 1970-1976 (K0-K6)
The 76-'78 F bikes (F0-F3)
...and the 77-78 (K7/K8) bikes
Many parts can be interchanged between groups, others cannot.