The only difference is age, we've found the older instruments take more time to calibrate than the newer ones. Plus, some of the components do age and they make the job harder. We try to keep as much of the original electronics intact while we engineer the solution.
In the age ranges above, the only significant date is 1975, when a revised version of the electronics was introduced.
Not every one needs the upgrade (seems that about 30% don't) and it is highly location specific. It may be perfectly quiet in one area, but when you play in the next town, you may have the directional noise, it just depends.
Also, the prices are maximums. If we're lucky and things go smoothly, it will cost less. Of course, if it takes dad 80+ hours, the price won't go higher.