On the other hand you can design a science vessel to be able to go by themselves and enough equipment to survive an encounter. That way you can survey gravitational points and new systems allot faster. These ships can also double as military scouts when needed in a war.
In most of my play-throughs I would not loose many survey cruisers that way unless they come up against something way more advanced than themselves, but in that case even a small military escort might get smoked as well. These ships are not that expensive either, around the cost of a regular frigate scout.
The initial survey cruisers would only survey some planets depending on the type of system. The point being for strategical reasons.
After this unarmed commercial GEO survey ships would take over and survey systems as I want to start exploiting them once they are secured and beacons dropped at all warp points.
There are little reason to completely survey every system all the time immediately. I tend to completely concentrating on GRAV survey to map the surrounding so I can plan my colonies and need for the infrastructure. Finding worlds to colonise, fuel sources to harvest or mineral rich planets with large reserves at decent accessibility is priority.
Keeping a dedicated GEO ship following along does not make allot of sense. I often end up putting a small GEO surveyor in a hangar on the exploration cruiser to survey some close by planets as the cruiser survey the system.
I also find that small ships tend to be rather slow so I rather build them with big fuel efficient engines and put a few GRAV sensors on them and perhaps then one GEO sensor or a shuttle with a GEO sensor. This means that the ship is fast and very fuel efficient, they also easily can fit a bit more sensory systems without compromising fuel economy that much. Some survey cruiser can be like 50-70% engines at times depending on technology level and what other system I want on there.
Early GRAV surveyors often are allot smaller (while a naval yard is growing) and I always survey like there are NO threats out there until I find a threat. The people making the decisions don't really expect dangerous aliens out there until they actually find any...
...I hardly have a military until there is a real need for one either for the same reasons.
Using build points and cost is often a very bad way of reason as that does not say if a bigger cost with yield a bigger return down the line. The cost of increasing the size of a yard also mean little as a yard can be used for other things down the line. You are not going to continually churn out survey ships all the time.
In C# you will make them all military anyway and small engines will not be very fuel efficient any more, so you will want bigger surveyors going forward. It is also easier to exploit good commanders for survey bonuses on bigger more capable ships in C# as well. These ships will survey much faster than several small ships.