Each time the costs increase by 15 percent. Keep in mind that the costs grow exponentially. So after five upgrades the next upgrade will cost more than twice as much as the first upgrade.
|Added Researchers||Price Increase||Example|
|+1 Researcher||1.15x price||50 → 57|
|+5 Researchers||2x price||50 → 101|
|+10 Researchers||4x price||50 → 202|
|+50 Researchers||1024x price||50 → 54183|
A table for each level can be found at Research Costs