I am at a loss as to how people can think putting both geo and grav sensors on a ship could be more efficient.
Consider a dummy system that masses 5 HS and costs 100 BP. If you put it on a dedicated grav survey ship, it will move slower and cost more. A ship that is grav surveying can't also geo survey, and vice versa. All the time that a ship is doing one task, the other survey equipment is dead weight.
The extra cost in shipyards, retooling costs and time might very well be enough of a factor to make it worth the hassle of some few extra fuel costs for "dead weight".
Using a single ship for many things is a strategic consideration and not about mathematical efficiency in one single instance.
Let's say you run an exploration vessel that also incorporate sensors and a small hangar with possibility of either some scouting vessel or perhaps a GEO survey shuttle. the point being that the ship are suppose to operate on it's own for an extended period of time.
One ships role are basically to do the grav survey and only geo survey any interesting terrestrial or potential fuelling spots, any dedicated geo survey will come much later if you deem it interesting. You are NEVER going to be able to exploit things in the same pace you can survey new space anyway so no real need to keep pace with the geo survey entire systems anyway, just the most interesting stuff. In these cases geo surveys are done much quicker than grav survey so having a dedicated ship for that (outside s shuttle) are probably pointless and a waste of fuel for the time being.
The point is to find the good strategically important systems and plan your general infrastructure, where to build the gates and where to send your real geo surveys and survey teams later.
Bringing a whole fleet for the purpose of scouting is also dead weight if one ship can find and potentially detect and avoid an enemy given they have the tools they need. It also means these ships can be drafted into military scouting when the times comes.
Any military ship that is not out doing military stuff is also dead weight in much the same way. So it certainly is not dead weight to bring along sensor equipment and weapons either.
There are allot more to consider than pure ship for ship cost for ONE specific purpose, the same goes for every ship that you create. The possibility of ships to multi-purpose in many roles simply make your strategic decisions far more flexible and overall less resource and time sensitive.
The only way a ship dedicated for one purpose make sense is if they can do that one thing none stop without wasting time moving about too much. In some cases I would agree a single purpose grav or geo survey vessels make sense too. But if your intention is not to geo survey every body in every system you encounter all the time it makes less sense to send more than one ship. You are never going to perfectly time when a grav or geo survey finish in synch anyway either.
So... calculating efficiency is almost impossible as it all depends on the situation and doctrines.