PGFPlots allocates storage inside of its axis environment as follows:
For every plot, it maintains meta data which does not vary much from plot to plot (unless the number of supplied options differs extremely). The meta data also contains
cycle list information.
Aside from this constant overhead, the current versions of pgfplots store the coordinates for each plot. This includes the x, y, z (if any), and meta data (color data, if any) coordinates by default. Each coordinate is stored as floating point number, and any digit is stored (which scales linearly in the number of digits you provided).
I think a good estimate is to take the number of points, the number of coordinates to store for each point (2 for a 2d plot without colordata, one more for a 3d plot, and one more if color data is needed), and the number of digits for each coordinate. That what indentifies the mem usage for a plot with
no markers style from the perspective of pgfplots.
However, pgf also allocates memory: it allocates the low level pdf stream data needed to typeset the individual lines. For a line plot (straight lines, nothing smoothed) without markers and without individually colored segments, this will boil down to two fixed points numbers plus a constant number of characters (parenthesis, commas, one char for the line-to operator). The fixed point numbers will have between 3 and 11 characters.
Note that this system-level-buffer of pgf also stores any drawing commands for the axis. This can be considered to be "constant overhead" in most cases - it won't influence the suggested resampling control, I guess.
I hope this answers the question deep enough. I think pgfplots mainly consumes the "main memory size", but if you need more information, one might need to profile the other memory types of TeX as well.
But as already mentioned in my comment above, I would rather suggest to increase the main memory size (the pgfplots manual contains detailed instructions how to do it) than to attempt semi-automatic down-sampling just to make TeX happy (especially considering that the system default of the main memory size varies between TeX distributions and will probably change in the future anyway).