The basic Soyuz booster design is a direct descendant of the R-7 ICBM used to launch Sputnik in 1957. They've refined the design over the years, but the basic Soyuz was largely unchanged for decades. That lowered the R&D costs to practically nothing. They also built them in huge numbers. When you add up the SL-04 (Soyuz) and SL-06 (Molynia - Soyuz with an additional upper stage) designs, some 2,000 of them have been built and launched since the early 1960s with a reliability factor of over 95%. The reliability is a lot higher if you discount the first 20 or so development launches. They literally launched several hundred of them in a row without loss. They're probably the most economical boosters ever built and one of the most reliable even if it is a relatively complex rocket.

The Russians have done the same thing with several of their other boosters. Sure, low labor costs are a major factor, but stable (unchanged) designs built in large numbers result in low costs per mission. Of necessity, they're also built to launch in more severe weather conditions than most American rockets, with the possible exception of our ICBMs and SLBMs.