IIRC, cost formulas are as follows:
Low cost: cost*level
Med cost: cost*(level^2)/2
High cost: cost*(level^2)
Since most things start at either level 0 or 1 (already researched), they work out to the same amount at low and medium tech cost:
Propulsion--research level 2
Low cost: 10000 * 2 = 20000
Med cost: 10000 * (2^2) / 2 = 20000
High cost: 10000 * (2^2) = 40000
By level 3, the difference shows up:
Low cost: 10000 * 3 = 30000
Med cost: 10000 * (3^2) / 2 = 45000
High cost: 10000 * (3^2) = 90000
Level 9 Stellar Manipulation will cost you either 900K, 4.05M, or 8.1M! Gets pretty pricey there. Anyway, hope that ramble explains/answers your question.
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