Actually, the trick for building geo- and grav-survey ships in the same yard is to create a third class with all the survey sensors either/both of them use, and tool the yard for that.
As Iranon mentioned, the problem is the high cost of the sensors relative to the rest of the ship: refit costs are calculated as the cost of new modules plus a surcharge for any change in ship tonnage, but removing something doesn't cost a thing. So refitting from geo-survey to grav, or vice versa, is way over 20% of the BP cost of either; but going from the hybrid to either of the specialists, all you're paying for is the change in size, which is peanuts by comparison.