Minor mining mechanic manipulations

[APARTHEID]

A minor change suggestion: Halve the price of the mining rig teleporter upgrade and make higher tier superdense asteroids more common (good luck finding more than one that's not maux/mol/phas in a round).

Superdense ore generation wasn't half-bad on Shroom Mining; with the rearrangement of ores with Cogstation, these extractor squares have dramatically decreased in number.

Half of the little four-squares around space are hardly dense at all! There's about two deposits currently on map if I recall correctly.