There are at least two arguments in favor of ability to keep Russian launches cheap(er). First if mentioned in John Clark's "Ignition!", where he says that the

There are at least two arguments in favor of ability to keep Russian launches cheap(er). First if mentioned in John Clark's "Ignition!", where he says that the Russian approach to bigger payload isn't the technology, but just the size of the rocket - with same low-tech as something smaller. So, in other words, technological edges aren't always translating into lower costs in this industry. Second is Russian's technological edges with kerosene liquid fuel engines. Technologies are well-optimized, including costs, and are the primary used ones for Russian space program. Not solid rockets, not hydrogen - and comparatively little hydrazin-based rockets, at least by the number of launches.