I believe the costs were approximately like this:
-Low: The cost of the initial tech level is added to the total for every tech level.
-Medium: Twice the cost.
-High: Thrice the cost.
I'm fairly certain about Low, but I have serious doubts about Medium and High.
__________________
O'Neill: I have something I want to confess you. The name's not Kirk. It's Skywalker. Luke Skywalker. -Stargate SG1
Low follows a linear progression. (10K, 20K, 30K)
High follows a quadratic progression. (10K, 40K, 90K)
Medium follows a quadratic progression as well, but all tech levels after the first cost half price. (10K, 20K, 45K)
__________________
The Ed draws near! What dost thou deaux?