I realize this isn't a math forum and that it's puzzling (it's not a pun), but I have these most reasonable answers by mathematical methods (regressions): (I'll also put in less math-y definitions)
All three equations appear to be power functions, or those of the form $y=ax^k$, where $y$ is the upgrade cost, $a$ is a constant, $x$ is the number of upgrades bought, and $k$ is another constant. $y = ak^x$ is another close fit, or an exponential function, following variable definitions above.
Note: correlation coefficient represents how close a regression line/curve (line/curve of best fit) is; $R^2$ close to 0 means almost no fit (or points everywhere) and $R^2$ close to 1 is really good fit, or points are really close to the line of best fit.
First equation: $y = 2.0858x^{1.6732}$, correlation coefficient $R^2 = 0.996$
Another good fit: $y = 3.464(1.5265)^x$, correlation coefficient $R^2 = 0.987$
Second equation: $y = 8.9852x^{2.0488}$, correlation coefficient $R^2 = 0.995$
Another good fit: $y = 19.943(1.6244)^x$, correlation coefficient $R^2 = 0.993$
Third equation: $y = 1.6153x^{7.2444}$, correlation coefficient $R^2 = 1$
Another good fit: $y = 51.982(5.1434)^x$, correlation coefficient $R^2 = 1$
187,096
meant to be187.096
or187096
or187, 96
? (Same typo[?] appears at math.stackexchange.) $\endgroup$