Like beauty, the term "low" in reference to SNs is in the eye (this case, mind) of the beholder. You have to take into account the total production of the weapon in question or, in the case of current production guns, the total production so far. Also, you have to know where the SN started (it didn't necessarily always start with 1). IIRC, during WWII, there were a bunch of companies making 1911s under government contracts. Each company was assigned SN blocks. An additional consideration would be changes in SNs over the years. You can judge a relatively recent S&W M10 revolver, for instance, without knowing how the SNs went over the years. The numbers initially were strictly numbers, but in more recent times became alphanumeric. Again IIRC, this was a mandate from ATF (I don't know why). It didn't necessarily start at 1, go through to 999,999 and then proceed in order from A0000001 through to BCX49924. To many variables.
For my own opinion, I would say the first few thousand (5000 is a nice round number) would be low SN.