You can flip your evaluation around the other way (i.e., start with the high range and go down).

=IF(HowManyAddlLots>9, 10, IF(HowManyAddlLots>0, 25, 0))

What this says is,

If > 9 then 10

Else

   If > 0 (i.e., 1-9) then 25

   Else 0

Because it checks for > 9 first, nothing will make it to the second IF unless it is 9 or less, so you don't need to check for a less than 10 value, you can just check if it is greater than 0 at that point.

You could also evaluate from low to high, the key is just that you don't need to check for < 10, you only need to check for > 0 and > 9.

For example,

=IF(HowManyAddlLots>0, IF(HowManyAddlLots>9, 10, 25), 0)

Here it is doing the following instead

If > 0 then

      If > 9 then 10

      Else 25

Else 0