It's actually an increase of 4 points per hour from 106 to 110, but I agree that a four point increase on a very nice planet (less on others) probably isn't worth the cost in research points or credits. The jump from level 3 to level 4 buildings is decent but rather expensive at 500K credits (plus 10 Erixion) per building. Still worthwhile in my opinion, and level 4 to level 5 may be worth the cost on nice planets, but I personally don't see much use in upgrading planet resource production structures beyond level 5. Higher level attack and defensive structures are more effective and don't seem to have a point where more research levels are not useful, particularly since those buildings are researched as a side-effect of gaining new levels of weapons and defense for the ship.

edit:
My own research is fairly balanced based on what I feel like I want next, with an emphasis on energy. I just finished Singularity Control (level 10 Energy), and have finished level 7 weapons, level 5 shields, level 4 cloaking, level 2 armor (I don't use it), level 5 defense, level 6 scanners, and level 5 planetary construction. I need more energy (always), more scans for better planets (there are about 25 planets in my scan list of 105 planets that are not complete garbage, all are occupied - mostly by me , and only 4 are really nice, so I need a lot more!), more defense, and more weapons. I don't think I'll bother much more with shields, armor, or planetary construction any time in the near future since I won't use the bigger planet buildings and higher shields and armor are actually a penalty and not a benefit until the formula used to calculate battle damage is changed.