[WS:] This is probably true, the cost would have been high no matter who was in power. Let's call it "necessary violence" for analytic purposes. However, it seems that Stalin added a fair amount of discretionary violence to the mix (cf. the Moscow trials), raising it well above what was necessary. Ditto for Mao (cf. the Cultural revolution). We should not sweep that discretionary violence introduced by particular leaders under the rug of the violence that was necessary to move the revolutionary project ahead.
Wojtek
Marx did say, "Force is the midwife of every old society pregnant with a new one. It is itself an economic power. " , though he was discussing the primitive accumulation of capitalism , not the transition from capitalism to socialism.
http://www.marxists.org/archive/marx/works/1867-c1/ch31.htm