please help me balancing mines and making them more useful

[Suicide Junkie]

I'm a solid supporter of the "eliminate sweepers" theory.

I also like the idea of having a divergent cost/effect ratio.
Cheap mines would be easy to build in bulk, but do relatively little damage each.
Expensive mines would take much longer to build and do much more damage.

The expensive mines would have a better damage/mine ratio, but a much worse damage/cost ratio.

The cheap mines would be cheap enough that you could fill 5 or 6 sectors for the price of one sector worth of expensive mines, so that multiple-sector fields may actually get built.