Rhenium is the major resource used to build your empire. The more deep the mine gets, the more it produces. Same logic goes for the energy needed though. As there is plenty of rhenium on your planets, it has the weakest value for trades. Developping your “Improved rhenium production” technology will increase your production.
Cost compared to levels
Cost of a rhenium or selenium mine : (level 1 price)*1.5^(n-1) (n beign the desired level)
You want to build a level 10 rhenium mine. According to the formula, it will cost you :
(level 1 price 1)*1.5^(10-1), which is, for rhenium : 60*1.5^9 = 2306 and for selenium : 15*1.5^9 = 576