Edit line items via script

I have a script written to modify a sales order upon save to do the following:

Check each line item for a condition and upon satisfying the condition, modify specific line item price level.

So far, I've been able to successfully identify the condition and set the price level and rate (kind of), but am stuck at a couple points.

1.  I want to perform a calculation on the base price of the item.  For example, Item A has quantity=1, a base price of $65, and price level of 2% discount for a rate of $63.70 and amount of $63.70.  After testing for the condition (and satisfying), I want to change the price level to 10%, rate to $58.50 and amount to $58.50.  I cannot find a reference for base price and have only been able to "back in" to the number via hard-coded values.  By doing this, I am able to set the rate...although rounding is a problem.