Instead of trying to determine the formula that was used for a (hopefully) complete set of numbers with unknown meaning, why don't you turn the problem around?

Ask the client create a new dummy lease with known input data. Tweak one (and only one) of the parameters and create another new lease. Repeat...

Given enough dummy data (known inputs and outputs) you should very quickly be able to determine which numbers are meaningless, which are significant, how each number affects the final value, and which numbers in the dumped data you need. Add a little algebra to get the formula, and you're done. This approach could be quite a bit simpler than trying to brute-force it. :-)