Balanced System Development By Era

System Developments are great to have, though currently aren't very balanced with a high resource cost in early game that makes them unlikely to be used for quite a long time and when they are used they provide a huge bonus.

Expanded To All Eras

To coincide with the next point it would be good to allow 5 instead of 3 System Developments by each era to create a more balanced approach.  It also allows for a research option for each era and a construction option, as there is a shortage of both.

Weighted By Era

Instead of spending 35 resources for a 50 point bonus, to have a cost related to the era and the ability of an empire to pay that cost.  This is key because of the lack of ability for most empires to construct these early.

For example...

Era 1 costs 10 resources for a +15 boost.

Era 2 costs 20 resources for a +30 boost.

Era 3 costs 30 resources for a +40 boost.

Era 4 costs 40 resources for a +50 boost.

Era 5 costs 50 resources for a +60 boost.

Or if still staying with 3 instead of 5 System developments...

Era 1 costs 10 resources for a +15 boost.

Era 2 costs 20 resources for a +30 boost.

Era 3 costs 40 resources for a +60 boost.

Clearly the numbers are examples that would require testing.

Built With Effort

Currently they are built for little production cost and time, which is fine enough though it is not much effort other than gathering the resources.  The production should relate to the era and should be equitable to that eras other production costs- probably at a better rate, say half or so the typical production cost for that era.

In Closing / tl;dr

System Developments are very expensive in resources to build, especially the early game, they are built too quickly and require little research effort.  Changing them to cost less resources and provide less bonuses early and to have more options to build them through all eras will help make them be more balanced.

The numbers plugged in above are generous for the resource and bonus, having said that they are roughly as good as currently used.

2cents :)