How can I calculate the cost of a business upgrade in Adventure Capitalist?

I have been using the coefficients at the Adventure Capitalist Wikia site page on businesses to plan my strategy, but either the coefficients are just approximations, I am calculating something incorrectly or possibly both.

For instance, I currently have 400 Giant Lasers on the moon. In the game, it says that x100 more will cost 6.68 Untrigintillion (6.68e96) but my spreadsheet (using the coefficient of 1.5) calculates it as 22.22 Duotrigintillion (22.22e99), which is quite a difference.

I can get it to calculate 6.68e96 if I replace the coefficient of 1.5 with 1.47572, but then my calculations for +1 and +10 are out. Ditto if I goal seek the value of the coefficient for other known values:

Giant Lasers    Actual Cost   Predicted cost
Bought  Wanted    in game     Coef=1.50  Coef=1.4699  Coef=1.47041855 Coef=1.47572
400     401       8.22e78      27.32e81    8.22e78      9.47e78        39.96e78
400     410     931.16e78       3.10e84  931.16e78    931.16r78         4.03e81
400     500       6.68e96      22.22e99  937.32e93      1.12r96         6.68e96    

The way I calculate the cost of the upgrade is as follows:

InitialLaserCost*(1-LaserCostCoef^LasersWanted)/(1-LaserCostCoef)
 - InitialLaserCost*(1-LaserCostCoef^LasersBought)/(1-LaserCostCoef)

^{Sadly we don't have MathJax here on Arqade}

I.e. The total cost of the number of Giant Lasers I want, minus the total cost I've already paid for the current ones.

As the difference between Wanted and Bought increases, the second term becomes insignificant, but it is good to confirm that when you want to buy just 1 Laser, the above matches up with the single term cost:

LaserInitialCost*LaserCostCoef^(LasersBought)

So, what (if anything) am I doing wrong, and how can I accurately predict the total cost of a multiple business upgrade?