Actually you guys are wrong. It was originally priced at 100 Munis each because they were quite awesome with the first set of stgs. The second set of stgs are supposed to be cheaper because they are lousier. You almost always get a death in the storm squad, reducing your firepower effectively, hence the 10 Muni cheaper thing. You can search for the forums for that argument. We originally wanted it down to 80 Munis or so.
Semantics.
The point is that it was to reduce the price in the STG stormtroopers, be it two or four, just like a negative example of this would be increasing the price of double panzerschreks, but not single panzerschreks, except in this case it was lowering the cost of 1, instead of raising the cost of 2.