I'm opening a new thread as it doesn't fit properly in my other one about Infrastructures. 

The technology cost is calculated as follows:

tech cost = base value * (1 + 0.01 for each previously invented technology).

Technologies are divided into smaller groups. Each group (technology level) has the same base cost. There are usually 3 such technology levels per era (except the last era with 4 groups).

In the game we have 90 techs so the last technology costs 89% more then the base value.

This is the base cost distribution over the eras:

In general, it is worthwhile to research aggressively (belining) in order to reduce the technology cost. Maybe you have other informations about science in Humankind.