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

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